Berkuta is correct, your first order of business will be to prep the inside of the buttplate. It would be very premature to go pounding on it at this stage, if you feel it needs any persuading then save that for the very end of the fitting process. Now too is a good opportunity to dress the outside of the plate, but that can wait if you wish. Before you begin the fitting mark centerlines on both the wood and your plate.
1. File the inside as flat and square as you are able. From your photo I can see the the forward end of the butt plate extension needs filing so the it sits level on the wood - be sure the top of the plate is inline with the comb of the gun.
2. It looks too as if you'll want to remove some wood at the toe of the plate to aid in leveling it. this is normal...
- You will need; metal files, rasps, bench chisels. You can use the chisels to scrap too. I use 1/4, 1/2 inch chisels, both a rough and fine rasp.
3. test fit often, go slow and think things out as you go - blacken the inside of the plate, put it in place and gently tap it with a non marring light hammer.
- remove the plate and then remove the blackened areas with whichever tool(s) you feel will do the job.
Repeat, repeat, repeat... eventually you will have learned a lot, have the techniques figured out and have only a little bit of butt material to remove. Slow down, test fit often, get it as close to perfect as you can.
The goal is perfectly mated surfaces all around the butt plate. The bit at the end of the process, that final fitting, is generally the most niggling and you'll be heartily sick of butt plates by then. Now
might be the time to do a bit of hammer adjustment to fully mate the butt plate all around, doing so is not always required.
But you can do it!
dave