Bill, it's as simple as can be! (Maybe even simpler! :thumbup: )
For the backing plate for a pocket sheath I use a piece of 6-7oz leather with the smooth side facing IN towards my leg. (For my regular sheaths the backing plate is made with thicker leather.) This puts the rough side against the cloth of the pocket so friction helps keep in in place. The backing plate needs to be cut just 1/8 inch smaller than the pocket (below the opening) so it can't shift around. Use the pocket for a template and trace around it, then cut the leather just a wee bit smaller. It's important to skive the front edges (rough side) of the plate so that they bend slightly inwards towards your leg. This will keep the plate from imprinting and being visible. (Without this, it would look as if you have a paperback book in your pocket!) all of this should take about 5 minutes to prepare... Nothing fancy here.
I make the knife pocket itself using a soft but thick suede, and it goes onto the plate on the side closest to your leg. (That's important! More suede and more friction to keep the sheath anchored inside the pocket.) I like my knives to be closer to the side rather than the front of my leg, so that's where I sew the 'loop side' Velcro to the backplate; almost to the edge of the backing plate. (The sheath pocket itself really just keeps the knife vertical and keeps it from shifting around. No need for it to be too snug. It's really just a squarish tube! ) I stitch it all around the knife, leaving a wider, almost 'funnel' mouth on it, and use a stiff piece of 'hook side' Velcro stitched and glued to the back. I then use a bit of white glue to stiffen the funnel mouth of the sheath just a bit. This makes it much easier to re-insert the knife when finished with it. If you prefer, you can stitch the sheath pocket directly to the backing plate.
The entire construction takes less than an hour, uses two pieces of leather (one 6-7oz backing plate and one 4-5oz suede for the sheath) slightly smaller than the pocket, two pieces of Velcro, and about 2-3 yards of stitching cord.
Tools - knife or scissors to cut the leather, skiving knife if you have one or just use a razor knife to thin the edges of the backing plate, a stitchawl, and a harness needle.
Keep in mind that you might have to make 2-3 different backing plates... Different pants have different shaped pockets and to make this work well the backing plate has to fit snugly. After a couple of wearings, the backing plate will bend around and conform to the curve of your thigh, the skived edges will bend in even further (you can wet them a bit and curve them in by hand if you want) and the entire sheath will be quite invisible in your pocket. Have fun!
Stitchawl