There really isn't that much science to stropping. Rub some on there if you are using the crayon kind of compound (that comes in little blocks). When it looks all dark, loaded up, or just doesn't feel like it is doing as much work as before you can either scrape off the loaded up compound with any number of implements or you can throw some WD-40 on there and wipe it away with a rag which is usually what I do.
I used to pull out sand paper (which is still good every once in a while if you are looking to add some texture or refinish the leather) but I don't want to be eating away at the leather and need a new strop or even worse...end up with a dished out or uneven strop because I am not sanding perfectly evenly.