I have some experience using French stitching clamps (pince à coudre) that I hope will help others.
In order to properly use this stitching clamp, you should position the clamp between your legs at a slight angle. Rest the hinged jaw on your left thigh (if you are right-handed; reverse the instructions for left-handed). The foot of the clamp must be firmly on the floor. The stationary jaw and the weight of the rest of the clamp (plus a little bit of additional pressure applied to the clamp by your right leg) will easily hold any item in place between the jaws. Orientation should be somewhere around 20-30 degrees off of vertical, so that you can more easily see both sides of the item to be stitched. Any more vertical, though, and the item you're stitching will likely slip out because there won't be enough applied pressure to keep the jaws clamped together. You won't be able to keep the jaws firmly shut, solely by squeezing your thighs together; it's just too tiring. The stitching clamp must be tilted. However, too horizontal an angle and there will be excessive pressure on your left thigh, making it very uncomfortable.
This means a couple of things must be taken into account simultaneously, for optimal use: 1) the actual, total length of the clamp; and, 2) the height of your chair or stool.
If you tend to vary where you sit when working, and your work chairs are of wildly differing heights, your stitching experience will suffer. So, instead of looking for a higher chair, as suggested in another post, just select a comfortable seat and standardize on that height for hand-stitching. Then, the next thing to do is to CUSTOMIZE your stitching clamp by cutting off small pieces from the end of your clamp (maybe 1", or even 1/2", at a time), and continually test the "feel" of the clamp until it seems to be the right length. The clamp probably is already angled at the bottom end, where it rests on the floor; this angle should be maintained with each cut.
At the correct length, you should be able to vary the pressure on the jaws by simply opening or closing your legs slightly. The bottom end should be wedged against the floor and sticking out to the side; maybe a foot or so to the right of your legs (again, only if you happen to be right-handed). Don't be too quick to shorten the clamp excessively; it will be tough to make it longer again should you cut off too much. Eventually, you can also pad the cut end by cementing a patch of leather to the bottom to help protect your floors.
I actually had 2 chairs of different heights that I used regularly (one was a folding portable stool for doing demos at exhibits and craft fairs; the other seat was exclusively for the workshop); so, I customized a separate French stitching clamp for each chair to make it easier on me.
Of course, once you optimize the length of a French stitching clamp for yourself, it might not be comfortable for anyone else to use. Though, maybe someone could design an adjustable-height stitching clamp to make it universally usable by anyone, of any size, in a seat of almost any height!
I hope this helps.
Note: Thanks to The Hub for the video on manufacturing a French stitching clamp. Very nice. Check out the section where the craftsman mentions that the clamp has to be tilted to keep jaw pressure on the item to be stitched.
