robertscott, just do them against the wall. That way, you can progress every week if you mark your position. I do them once per week, and almost every time I can improve. I always did one set at the end of a workout, and first I did 5-10-15 reps, and then I move away from the wall for a little bit, and then repeat 5-10-15. When I came to almost full extension I reduced to 3-5-7.
Read about them on beastskills...
Don’t run if you can walk, don’t walk if you can stand, don’t stand if you can sit,
and if you sat, might as well lie down and take a nap.