A good place to start would be with a book. Books on the various Briggs engines are readily available, and give you the info that you need for disassembly, diagnosis, and testing. Oil consumption can have several causes:
1) Piston rings worn out or bore scored.
2) intake valve seal/guide worn out.
3) breather restriction/malfunction.
4) Leaking externally.
Rough running is usually a side effect of oil consumption. Oil getting into the combustion chamber fouls the spark plug(s) and causes erratic firing or misfiring.
Some of these potential causes can be easily fixed while others can get expensive. Rings or bore problems usually mean a full disassembly and trips to the parts store and the machine shop. Intake valve seals are less complicated, and most leaking seals are relatively simple and inexpensive to repair.