Substance Painter - Normal Map

Hi Folks,
I have a pretty basic Question but couldnt find the answer till now. When im working with my Object in Substance Painter and i bake a normal map how exactly is the workflow if i would now love to modify this baked normal map (like adding scratches and so on). I know it sounds simple but i fond a lot of tutorials in how to fix baking artifacts but im really curious how i can actually add scratches and stuff. Sorry if thats a stupid Question any help would be appreciated