I used to have this problem untill I did the following:
Step 1: Get some quality leather, not too thick. Must be actual leather, pleather doesnt cut it. Test it to make sure its soft stretchy leather, dont use harder tanned leathers.
step 2: lay out the pants with the entire crotch/inner thigh area COMPLETLEY FLAT. No wrinkles. Use rocks to weigh the material down make this happen.
Step 3: Take a piece of paper and center it over the crotch seam, trace down the center line well down the thigh, mine ends 4/5ths of the way to the knee. Then make marks approx 1.5-2” on either side of the center line. You want the piece widest in the middle, to account for the tappering shape of most pants legs. So you end up with kind of an oval shape with the narrow ends flat.
You now have a (hopefully) perfect trace of the size and shape of your soon to be crotch patch.
Step 4: Trace and cut out the patch
Optional but recomended: Get a leather hole punch and punch holes in your patch around the edge. Atleast 3/16ths in from the edge. This makes sewing it on sooo much easier. In the past I’ve literally just bought one from some random craft store, used it and returned it.
Step 5: Sewing. I start by Tack sewing my patch on, starting in the middle. Just one or teo stitches then tie them off. Then doing 4 more at each corner. I’ve found that this reduces the sew time drastically for big patches because you dont need to fuck with it as much to make sure your sewing it flat.
SEWING IT ON FLAT IS KEY.
Sewing that isnt flat and strait will concentrate shear forces where the fabric bunches up, which will eventually tear out. Requiring more patches.
Now start sewing for real. Use a parallel running stitch, like another poster showed in pictures above. NEVER use a whip stitch. By thag I mean a stitch that is perpendicular to the patches edge. Whip stitches suck. They always form holes or rips at the edge of your patches. Banish them from your life except for temporary repairs or very specific applications.
Step 6: Once you’ve sewn all the way around. Turn around and sew back, poking the needle thru your previous stitches as much as possible. This serves two purposes, 1 It locks each stitch in place. You can break individual stitches without the whole thing unraveling. 2 it evenly distributes stress forces along the whole edge of the patch, instead of at each stitch hole. Dont skip this step, you will get holes.
In the end you end up with a leather reinforcing patch that is perfectly melded to the shape of the pants, they should move as one unit.
I started doing this a few years ago whenever I got bee pants. and I have literally not had to sew in the crotch area since. Infact the patch I have now I recycled from my previous pair and may just do that again for my next one. It’s that durable. Yes its alot of work up front, but looking at the dozen or more patches I out just on the crotch on previous pairs its worth it in the long run.