Caramelized onions

Melt butter in a large heavy bottomed pan over medium-high heat
Add onions, salt and sugar to pan. Stir to coat
Cook over medium heat for 15 minutes, stirring several times
Reduce heat to medium and cook for an hour, stirring about every 10 minutes (a heavy bottom pan helps here as it will reduce hot spots and promote even cooking)

Onion variety isn't of great importance, but if Vidalia onions are in season they are hard to beat.
Onions are mostly water, so you will have a lot less final product than you'd think. Twelve cups of onions to start with will turn into about a cup of caramelized onions. Adjust recipe accordingly so that you end up with the right amount for your needs.
The darkened color should come gradually, if you stir the onions and notice only the bottoms show color, your heat is too high or you should be stirring more.