I agree with both Mike and Smart Dog that English style butt plates are simple to instal. I don't time my work so I have no clue how long it took me to install the one I pictured in the previous post.
My second build from a plank I had done about 20 years ago. It is a Fusil Fin. That butt plate has the same situation that if you inlet the comb straight down and then go forward to fit the rest of the plate you will end up with significant gaps. I did a German butt plate about 2 years ago and had to revisit the fusil's BP to try and remember how I did it.
I did remember that I had made a good pattern of the inside of the plate before I cut it on my bandsaw.
I used Tracks catalog as a start for my pattern. Then I spent a lot of time figuring where the forward end of the comb would end at. I left the vertical part of the plate pattern on the wood fat. I figured I had to inlet the comb and the vertical part of the plate somewhat simultaneously. After I cut the plank I checked it to see how close everything would line up. By the way what I'm relating here pertains mostly of what I remember of the German BP I inlet a couple of years ago.
About the only thing that I remember of the fusil's inlet is that I made a pattern. Also that it took me about two profanity laced days to inlet and didn't hammer it in.
I don't have photos of the German BP because it is in the process of being engraved by an engraver friend in New Mexico. But this is the one I did 20 years ago. The bp is held by the pin on the comb and one screw in the lower third of the bp.