Abalone-wise, here's the 411...
First off, you want to find the area of the mother of pearl that you want to use, taking into consideration it's thickness relative to the intended use- you don't want a piece a half inch thick for an inlay unless you
really like grinding... :crutch
Melt some wax over the piece you want or over the entire inside of the shell- the purty part.
In a place that it won't matter, like out on the sidewalk of someone else's place

splash some Muriatic acid that you picked up from a pool supply place and step back while it fizzes. Do it again. And again.
Now hose the shell off and whip out your trusty angle grider with a very coarse disc. Wear a no bullshit respirator, goggles and then ask someone else to do it for you a half a block down wind. The important thing here is to do nice even passes over the shell realizing that the shell is in no way even in thickness so be cool and take a good gander at how things are going, often.
Once you've busted through the grunge outer layer and you're into the pearl, change to a finer grit and if you have a D.A. or an orbital sander, smoooth.
Once the thing is to your liking thickness-wise, put the shell in some boiling water to remove the wax, bust loose the piece you want and do the shaping and final final with a Dremel tool in a no bullshit well ventilated area where the wind is blowing the stuff away from you and over to the neighbor's yappy dog dish. :head
To the gun, trace out the design and go real shallow with the hobby blade and glue with some 5 minute or 2 ton epoxy and wonder why the balance of your gun just went to Hell. :waterwork
And thanks for sending those pics of your other gun, Tommy :hmm tick, tick, tick.