Rick,.... there's really not much you can do to "mess something up", as long as you can identify the lock you currently have and can find it's matching "mate" in flintlock.
Also,.... if your percussion gun has a "breech plug and drum", all that's necessary is to screw the drum out of the barrel and replace it with a vent liner that has matching threads.
If your percussion gun has a Patent Breech (w/integral snail) then the breech plug must be removed and the barrel "rebreeched" with a matching Patent Breech (sans/snail) that will accept a vent liner.
and finally, after first removing the "cock/hammer" from the new flintlock,.... you'll have to clamp the new lock in it's mortise, and "drill and tap" the lock plate for the lock bolt, but that's no biggy as you already have the hole through the stock to use as a guide.