About two minutes in the food processor, and... magic happens!
Put the peanuts in the food processor. Process for several minutes. At first it will look like very crumbly.
Continue processing. As the oil is released, the crumbly pieces will start to stick together, and then the peanut butter will clump together in a large ball as it takes on a paste-like consistency. Add oil if needed to keep it smooth, 1 teaspoon at a time.
Add sweetener and/or salt to taste and continue processing until the peanut butter reaches the desired consistency.