You will have to remove the skid plate to get to the filter. Loosen the rear two bolts and remove the front two and the plate will come off. Be sure the engine is warm before draining the oil, but don't do it if the engine is very hot (the hot oil burns!) The washer you got is the sealing washer for the drain plug. Remove the old washer and replace with the new one. You may have trouble removing the filter, many are very tight from the factory. Get a filter wrench from your local parts store if you don't have one. Clean the surfaces where the filter is installed, and be sure to coat the filter rubber seal with oil before installing. Don't over tighten the filter; follow the tightening instructions on the filter. Reinstall the plate, fill with oil, start the engine and look for leaks at the filter. Stop the engine, let set for a few minutes and recheck oil level.