You just make it flat. So that you are gluing a flat piece to a flat piece. The 2 pieces will have to have an absolutely tight joint surface otherwise you will be left with a not so nice looking line all around the new piece.
That is why sometimes (depending on the value of the gun, how bad the stock is, etc) it works just to fix it with Acraglas that is dyed to the same color that you will stain the gun.
The correct way to fix a stock is not always the best way. It doesn't make sense to send a stock to someone to get repaired when that repair will cost more than the gun ever can be sold for and for a gun that has no family/sentimental connections.
Unless you are very skilled at inletting it in, the grain is always going to mismatch.
I have one here that I fixed with Acraglas as well as others that I glued the chips back on (I had the chips).
In order to match the grain you are going to have to go to either use a bigger piece which ends up intruding into the cutout (which you will have to restore) or use a couple small pieces very carefully fit together.
Get the crack fixed first and the rest of the oil leached out before committing on a way to go.
here are some videos to help you make a decision
https://www.youtube.com/watch?v=AH4Akq1tUUUhttps://www.youtube.com/watch?v=77u1e2SsUQUhttps://www.youtube.com/watch?v=HiQxACLWIyQ