As for the Sissy Squat, the picture left shows it, but the picture doesn't do it justice.
It resembles too much of the front squat.
The main thing as I remember was keeping the hips forward and knees over feet. All too often people have mistaken the roman chair squat for the sissy squat - when they are nothing alike.
The sissy squat as an exercise takes on the form of a leg extension/squat more or less.
I started by performing these on his hack and/or smith machines.
I still prefer a fixed motion machine as opposed free weights - for greater control.
You may want to start without any weight, and just holding onto the smith bar for balance.
The first 5 reps are just the way those little images show. The body is straight from the knees to the shoulders and you simply lean back as far as you can, and then come up (make you a heck of a good limbo dancer, too).
The next 5 reps are from that lowest point you reach when you are doing the first 5 (so lean all the way back), and then fall to your ankles as if you were doing a full squat. From that full-squat position you don't stand up, but rather, thrust the hips forward to get back into that low position. Then back down to your ankles again.
The final 5 reps are the combination of both. Lean back as if you are doing a limbo. When you reach that low position fall down to your ankles, then thrust the hips forward to come into that low "limbo" position, and then come up.
That may or may not have straightened things out, not sure.