Pour the peanuts into a large food processor fitted with an "S" blade. Secure the lid and process until broken down.
Within about 2 minutes of processing, the peanuts should become a relatively smooth paste. When you process it for 2 more minutes, the peanut butter will become even more smooth, and you'll start to hear a "sloshing" sound as the peanuts release their natural oils, making the peanut butter have a runny texture.
Within 5 minutes, the peanut butter should be very smooth and runny. Add up to 1/2 teaspoon of fine seal salt, and process again until evenly incorporated. Pour the peanut butter into an airtight jar and store it in the fridge until ready to use.
Homemade peanut butter should last up to a month in the fridge as long as you don't introduce moisture to the jar. Always use clean utensils and return it to the fridge for best shelf life.