Place the roasted peanuts into a food processor and blend on high until they're finely ground up - this can take up to 3 minutes.
Add 1 tablespoon of peanut oil to begin with and continue to blend for about a minute. Add more oil as you see necessary to get the consistency you're looking for.
Add ½ tablespoon of powdered sugar and a pinch of salt and blend until combined. Taste it and add more sugar and/or salt to your liking.
Scrape into a jar and store in the fridge.
There are really no exact measures when it comes to making homemade peanut butter. You'll have to find out the quantities that work for you to get the desired consistency and taste, but the ones above are a great guide.