It's really quite easy. You need:
Peanuts. That's the only ingredient (they don't need to be super extra-large, but I went to Costco and that's what they have). As a side note, you want to check the ingredients to make sure there's just peanuts in there (you'd be surprised what some brands add to just peanuts). NOTE: You can use whatever nuts you want! Doesn't have to just be peanuts - mix it up (ha) and use mixed nuts or whatever strikes your fancy!
You will use:
A food processor, spatula, and jar. If you use a mason or other small-neck jar, you'll want a jar spatula (makes less of a mess).
Now for making peanut butter!
First, put the peanuts in the food processor.