1 frozen/thawed mouse each week, from pinkies to jumbos.
Dechlorinate tap water and replace often.
Corn snakes are very easy to feed and almost never refuse a meal. Firstly, the easiest way to gauge what size mouse or rat needed is by comparing the girth of the rodent to the girth of your snake's belly. The mouse or rat should be about the same size around, or a touch larger. If you feed them a mouse equivalent to this size, a feeding once a week is ideal. Because corn snakes vary in size, there's no exact food size to follow. A hatchling corn snake is best eating pinky mice, while adult corn snakes usually need adult or jumbo mice.
There are many ways to thaw frozen mice out, from leaving it out for a few hours, to putting it in a plastic bag in warm (not boiling) water. Do not microwave them, and if you choose to speed the thawing process up using a heat lamp or heat mat, keep a very close eye, because a burning, rotting mouse is something we can all live without smelling.
One of the nice things about corn snakes is the fact that they almost never outgrow mice, so you don't have to worry about switching them to rats which is sometimes difficult to do.
Simply hold the mouse with snake tongs to avoid your hands being mistaken for food and shake the mouse around the head of your snake until it strikes and grabs the rodent. Avoid sudden jerks with the food away after it's bitten. Pulling the mouse too hard could result in your snake losing a tooth, although this is rare. Handling your snake after feeding may result in a regurgitated meal, which is not only a waste of a mouse, but very demanding on your snake's body to vomit up.
Should you feed your snakes outside of their enclosure, like in a separate tub? I highly don't suggest it. "Cage aggression" is a term often thrown around that describes a snake who connects hands around its enclosure with food, therefor increasing the chances of being bitten. However, I have yet to see any proof of this being true.
How long you should wait after feeding depends on both who you ask, and on the snake itself. For example, I could handle some of my snakes a couple hours after feeding and they wouldn't care (although I still avoid it.) while others need a few days to digest. I suggest waiting 24 hours before handling, however, some people will tell you 48 to 72 hours. The more you get to know your snake, the better you'll be able to gauge this. A good way to ensure you don't stress the snake out is by feeding the him/her before you go to bed, allowing the snake to digest overnight.
A good rule of thumb is to purchase a bowl that is large enough for the snake to fit his or her entire body into, yet is small enough that the snake can get itself out of the bowl easily. Snakes can stay underwater for long periods of time, however can still drown. Tap water nearly always has chlorine and chloramine which can be dangerous to reptiles, however there are many ways to ensure your water is safe for reptiles. Filling a jug of water and leaving it out for 24 hours will allow the majority of chlorine to evaporate, although chloramine does not evaporate this quickly. You can also use "ReptiSafe" water drops to remove these chemicals.