Any comments or tips before I do my first one? I've also seen them installed backwards what is the purpose for that?
Don

If you're speaking of the hinged belly traps, the backwards-vs-forwards direction is largely determined by the smith's desired rear sling swivel positioning.

Almost all modern-made traps are hinged at one end, making them quite long in relation to the older side-hinged models and thus placing the sling swivel far, far to one end or the other compared to the body of the trap. This can become awkward at times and so the smith will position the trap's swivel end in the best way to suit the particular need.

Another consideration is the actual length of the cartridges or other items under the trap, and their relationship to the outside of the stock. You'll need to allow plenty of wood thickness over the cavities to avoid both the hollow sound of a too-thin barrier as well as the all-too-real possibility of breaking through to the outside when drilling the holes.

I have installed them facing both ways at various times and been equally satisfied with the results. If you put the hinge at the rear then the cartridge length is sometimes limited while if you put the hinge at the front then the (usually extremely thick but occasionally TOO thin) lid must be cut down to match the wood line at the stock's toe. Your particular stock and your particular trap may actually determine which positioning is best or even acceptable.

Tommy Bish, in one of the Home Gunsmithing Digests, shows how he made and installed one of these belly traps. Well worth the reading if you get the chance.

The very best suggestion I can offer is to keep your chisels RAZOR SHARP and to GO SLOW! Chipping around the edges of the inletting is one of the worst problems here.
Regards, Joe

Forgot to add that 'draft' can be critical when installing into an existing finished stock. A little taper is necessary to retain and preserve the tight gap-free fit at the top edge as the base portion is inletted progressively deeper into the belly, but too much taper can often result in a wedging effect that will split the thin wood at the top of the cut.

You can imagine how I learned this. BE CAREFUL!
Regards, Joe