Transfer your roasted peanuts to your food processor (or blender if its powerful enough). Blend until it turns like powder.
Once you reached a fine powdery consistency, add your oil. (Avoid oils with strong flavours because your peanut butter will taste weird)
Blend again till it forms a paste. You can season at this stage with salt or honey if you wish. Personally, I don’t feel the need to add any seasoning. Transfer in an air-tight jar.