Oatmeal is a tricky one -- you will get lots of opinions on this.
First, I would only use the instant flaked oats -- they are essentially 'pre-cooked' so they don't require any additional boiling, like you might have to do with other forms of oats (be sure to check at the grocery store, because there are many different kinds: instant/quick, flaked, rolled, etc.).
I prefer to use oats in a mash of some sort (e.g., a partial mash if you are not an all grain brewer). If you mash oats with a base malt that will release enzymes to convert starch to sugar, then the base malt will convert the starch in the oats, leading to a clearer and somewhat better tasting wort.
Here is a good article on the partial mash procedure, including a recipe and discussion of using oats in a stout:
http://***********/feature/1536.html
Having said this, if you are doing a stout, then some say don't bother with a mash because a clear beer is irrelevant, and starch haze doesn't matter (stouts are just too dark to begin with). So just steep the grains in hot water (150-160F) for a half an hour with your other specialty malts, rinse, and add to your boil. This can all be done in a pot using a grain bag or strainer to separate the liquid from the grains. It takes about half the time, requires less attention, and less equipment.
Here is some great info on steeping:
http://www.howtobrew.com/section2/chapter13.html
Sorry that wasn't a simple answer, but I hope that info helps.