I have a Colt New Service. The grips on those are punishing -- a few cylinders full will leave you with a sore knuckle.
The only grips I could find for sale were repros of what I already had. A friend who does woodworking had some scrap walnut, and he kindly used his bandsaw to cut a few pieces to a cardboard template I had drawn.
I put the gun down on the blanks and carefully drew the outlines of the area to be inletted with a pencil. Then I used a 1/8" veining tool (about $10 from carving shops) to cut a "trench" along the pencil lines.
I have an old Dremel tool with a stand. I chucked a router bit in it and carefully adjusted the depth of cut. I would route out an amount equal to about the thickness of two sheets of paper at a time.
After fitting the grips to the gun, I shaped them with a bench sander, a Dremel rotary sanding bit and a shoe rasp.
I made them wide and hand-filling -- as you can see. Just the thing to tame recoil.
You can also see a gap or two -- I saved dust from the sanding and routing, and will mix that with epoxy and glass bed the grips later.