With a 70+ gear ratio, you aren't strong enough to lock the rear wheel with all your body weight on it. You have to temporarily remove weight from it, either by leaning forward, or some other type of body finesse to initiate the skid. A good cyclist should be able to temporarily hop the rear wheel from the ground, even without leaning forward much....sort of like a tiny bunnyhop of the rear wheel only. Once you can do that in a seated or near-seated position, you do basically the same thing to initiate a skid, but you don't actually have to pull the wheel completely off the ground.
Once the skid is initiated, the friction coefficient decreases, so it is easier to maintain the skid once your weight falls back onto the rear wheel.

In a skip stop you do this repeatedly in cycles....unweight-shortskid-unweight-skid-unweight-skid-----etc.
Its very easy learn to do this seated, or standing with ass above the saddle, but it takes a lot of strength, practice (and a lower gear) to really hold a seated skid.
If you just want to skid continuously, then just shift your weight as forward as possible (nuts-to-stem) to keep weight off the rear wheel. But thats not going to help you stop.

Edit: this all comes to me as second nature, and I don't think much about how its done. But I think I use the upward momentum of the crank during the backstroke by pressing against it (sort of like a spring) to help temporarily unweight the rear wheel without even having to leave the saddle. I'll think about this a bit as go out for a ride later today.