The explanation in Don Casey's "This Old Boat" was very helpful to me. The book should be the public library. If not, then any good boat maintenance
book should have a section on it with good illustrations.
Good luck. It is sometimes a very tedious project
but keep raising/lowering and adjusting port to starboard the fore and aft engine mounts until the couplings line up nearly perfrectly using a feeler gauge. It took me the best part of a day to get it to within proper tolerances.