From my recent studies I'd say start by letting a gallon of whole milk expire for a week. Then use the instructions from the mozzarella video I posted a few pages back (I'll re-post if you like). The spoiled milk seems to always end up making ricotta instead of mozz...
Lol, it has to do with the acid denaturing the proteins when they have been heated. You could follow
this. Though it's a bit involved. I think it's more a matter of temperature rather then acid type. Between 135-140f you get something stretchy like mozzarella. Around 165-185f you get ricotta. Though there is a gremlin in there someplace that sometimes gives you one when you wanted the other.
The short version is, heat your milk between 165f-185f in a non-reactive pot, then add vinegar and stir gently until the curd and whey separate. The whey will turn yellow, and won't be as opaque. This can take between 1/2 cup and 1 1/2 cups vinegar depending on the milk. Pour the curd and whey through a clean undyed dish towel in a colander.
Bunch up the dish towel around the curd, and rinse it under cold water in the sink. The acid level from the vinegar is going to be a little high, and if you leave it in there it's going to be pretty tangy.
Wait for it to drain, 20 minutes or a bit longer is what I usually do. Then salt, about 1/2-1 tsp for each gallon of milk you used. Stir it all up. Stick it in something like a tupperware container in the fridge.
Heating the milk takes the longest. You need to do that over no more then medium heat, and stir regularly. If you try to heat it to fast, or don't stir, you will end up with scorched milk on the bottom of your pot. It's a pita to scrub out.
If you use milk that has soured it's already got lactic acid in it, so you don't need as much acetic acid from the vinegar. It also is a little more reliable if you want ricotta. Fresh milk sometimes goes stretchy, I've never had that happen with sour milk. Sour milk also retains some of it's tang, in a good way, no matter how well you rinse it.