This is how I glaze to get a two-toned pot. One color glaze on the inside, and a different color on the outside. Some of this applies to glazing anything. Some of it is specific to my clay and glazes, and the particular effect I want to achieve.
First I wash all the bisqued pots down with clean water and a sponge. Any clay dust or greasy fingerprints can keep the glaze from adhering tightly to the pottery during application. They act as a resist to the glaze. You won't necessarily know there is a problem until after the glaze firing. During the firing, the glaze melts and moves over the surface of the pots. Dust and dirt encourage the glaze to run right off, instead of creating a good bond to the clay, leaving little bare fingerprints, or creating large, uneven surfaces where clay dust settled onto the bisque before glazing. Be very wary of using lotion on your hands on glazing days. (Sorry, I know pottering means chapped hands. If you're just dying, try to get the backs of your hands greased without getting any on your fingers.) It's important to keep a very clean studio for this reason, as well. I glaze and store my bisque in a separate room from the rest of the studio to cut down on dust problems.
Glaze is made of finely ground minerals mixed with water. The powdered sand, metallic oxides, and clays that compose a glaze all settle quickly over time and clump together at the bottom of the bucket. It is critically important to remix your glaze thoroughly each time you glaze a batch of pots.
Before each glazing session, I pour my glazes through a dedicated kitchen sieve into clean five-gallon buckets, scraping down the sides well with a spatula. That breaks up any lumps and remixes the glaze. Then I stir vigorously, seconds before each dip or pour. The glaze needs to be a completely even consistency from the top of the bucket to the bottom before each dip, or you will not get an even coat of glaze on your pots, and some glaze ingredients settle VERY quickly. Stir in between every pot.
For my glazes, something the consistency of heavy cream is just right for a perfectly glazed pot, but every glaze is different. It's important to know your glazes well, so that you can add a little extra water to the bucket as necessary each time you glaze. Water evaporates from stored glazes over time, particularly in a hot studio, but knowing how much water to add to make up for this loss is a question of experience. Because they have different ingredients, some glazes will need to be thicker, or thinner, to give you a perfect coat. Depending on how you are applying or overlapping them, you may also need a thicker or a thinner batch.
One of the most common problems for beginning pottery students is a tendency to use too many glazes. Instead of getting to know three or four glazes well, they want to use twenty, and they don't generally take good notes about their experiments. The results range from the mildly disappointing: ugly pots; to the disastrous and expensive: damaged kilns and ruined kiln furniture. Ever wonder why I can't make you that mug shape you like, but in purple? This is a big part of the reason. It's impossible to guarantee good results unless you know your glazes REALLY well. I've developed a few that I like, and I stick to them.
Next I pour a generous amount of glaze into the inside of the piece with a small pitcher, and carefully swirl the liquid to coat the entire inside. This must be done quickly to get an even coat, and takes a steady hand. Interestingly, leaving glaze sitting in a piece for too long before you pour it out can cause some really weird problems. The porous clay soaks up water from the glaze. That's what initially gets the glaze to stick. If you work too slowly, the clay will soak up as much water as it can hold, and then, being sopping wet, the minerals that actually make the glaze do it's thing won't stick evenly. It's like trying to wipe up a spill with a sopping wet sponge. Nothing sticks to the sponge, it just smears around. So work quickly.
I quickly pour the excess glaze out of the pot, and hold it upside down until the last drips fall off the rim. I don't want any extra drips to run back into the inside when I turn the pot right-side-up again, or they will spoil my nice even coat of glaze. Once the inside coat has dried enough to permit safe handling, I use a damp sponge to clean any drips off the outside of the pot and the rim.
I let the outside of my pots dry until I can't see where I sponged it clean. This can take several hours, so I usually glaze the insides of all the pots, sponge them, eat lunch, and then dip the outsides, starting with the pot I glazed first. It's important to be sure that the pot is evenly dry before dipping the outsides. Remember, when you apply a glaze, it only sticks because the bisqued pottery is porous. It soaks up the water from the glaze like a sponge and leaves a film of powdery minerals high and dry on the outside of the pot. If your bisque is already wet when you dip it, because you just sponged off some drips, those wet spots won't be able to absorb any more water, and you will get a slightly thinner glaze coat in those areas. If you accidentally drop a pot into a bucket of glaze, and need to clean it off completely and start over, (yep, it happens), you will probably need to let the cleaned piece dry overnight before you can start again. The thinner the walls of the pot, the longer the drying time. Thick pots are thirsty. Thin pots sip their glaze delicately.
There is one exception to this rule. Pieces that have a trimmed foot. Because of the way that trimming roughs up the clay particles, a trimmed area will absorb water and glaze differently than an untrimmed area. This can lead to little pinholes in the glaze on the trimmed feet and bases of your pottery. Some potters paint a layer of slip or terra sigillata over their feet just after trimming and give it a quick burnish to seal the surface back down. I'm not quite that persnickety. If my pottery is the right consistency for trimming, I can give it a quick burnish without the extra slip step, and then run a damp sponge over that section just before I dip the outside of the pot. For my clay, that does a good enough job of evening out the absorption rate of the clay and eliminates about 90% of the pinholes.
Dipping the outside of a pot can be a bit tricky. You need to find a way to hold it that doesn't get your fingers in the way. I hold small mugs by stuffing my whole hand inside and spreading my fingers. It lets me dip all the way up to the rim. Bowls with inward-curving rims can be held with two hands, just on the inside of the bowl, and also dipped all the way up to the rim.
Bowls with rims that curve out, however, require a few fingers on the outside of the rim to hold them, so you need to plan for this from the start when you are thinking about what you want your finished piece to look like. Either that first outside-of-the-bowl dip won't go all the way to the rim, or you will leave some fingerprints, and will need to plan for a way to cover them up later with a design element. You can also use specially made dipping tongs to hold a piece while you dip it, but they also leave a small, but visible, mark in the glaze of the finished piece, and can crack a large piece with a thin rim. The best solution depends a bit on the glaze and a bit on the pot. It's a good idea to plan your glaze design in advance to minimize or cover up fingerprints or tong marks. Some glazes are forgiving enough that you can touch them up with a paintbrush, and any marks will disappear completely during the firing. Other glazes are not so kind and you will need to put a polka-dot, a flower, a stripe, or something on that spot or it is going to draw the eye like that particularly horrible zit you had at the prom.
Next, I clean the bottom of the pot with a clean, damp sponge. When the pot is fired, any part of it that will touch the shelf it sits on in the kiln has to be completely free of glaze. Otherwise the glaze will permanently glue the pot to the kiln shelf. Glazes are liquid during the firing, but create such a tight bond as they cool down from the firing, the only way to remove a stuck pot is a hammer, and there is a great risk of ruining an expensive shelf.
Rinse your sponge frequently, and squeeze all the water out of it. It needs to be damp, but not dripping, or you'll make a smeary mess. You want to be able to remove glaze from the foot ring of your pot in a nice crisp line. That's one of the things that separates a great pot from a ho-hum one.
It takes a good sponge to do this. I like the big yellow grouting sponges that contractors use for grouting the tile in your bathroom. They have very fine, even holes. I cut them up with scissors to a manageable size, and throw them away as soon as they start to become ragged.
If you know your glaze well and you know your pottery forms well, you know exactly how much the glaze is going to run down the pot during the firing. Wipe back the glaze just as much as you need to, but no more. An extra ridge on your foot ring can help catch glaze drips, allowing you to glaze further down the foot. A properly balanced glaze will be docile and predictable.
Look at the commercially produced dishes in your kitchen. Do they have giant unglazed feet that scratch your table? Do they have drips with kiln wash stuck to them where a pot almost glued itself to the kiln? I doubt it. This is one of the things factory-made pottery gets right. There is no reason you can't too. The things that separate your work from commercial work should all be good things. They should be the things that you can do as an individual with thumbs, which they can't do with machines on a conveyor belt. You aren't working in a mold, so you can make complex forms with lots of curves in and out, and without visible seams. You can do complicated, multi-step glaze applications. But the factory should never beat you on craftsmanship! If their pots have neat little feet, yours should too. Think about these sorts of issues as soon as you start throwing a pot or testing a new glaze recipe.
This is one reason I often say no to special requests for particular pots. I can't predict how my glazes will run down a novel form and I can't predict how a new glaze will run at all. I evolve my forms slowly over the years and take lots of notes. I repeat forms over and over and over. Knowledge is the key to good craftsmanship and repetition is the source of that knowledge. If you want to constantly branch out in new and exciting directions, be a painter. Paint mainly stays where you put it. There are too many unpredictable variables in pottery to be a dynamo. If you want to be a potter, get disciplined and plod stodgily towards genius.
Finally, I dip the rims of the pots. Small pieces can be held easily by the foot, but large pieces can present a challenge. Again, it's important to plan ahead. When you are throwing and trimming large pieces, you can save yourself a lot of heartache later if you be sure to create a foot that will be easy to hold for dipping.
And voila! About 8 hours later, your load of two-toned pots will be all glazed and ready for the kiln...