A good sit spin is so far down the skater is almost sitting on their spinning foot. Dick Button likes the skater to keep a straight back for stylistic reasons, but it doesn't do anything for the spin technically. For one thing you can't go all the way down and keep a perfectly straight back. I prefer going all the way down. For the beginner the two hardest parts of doing a sit spin are usually going down more than a few inches and centering.
Beginners have a tendency to spin on an outside edge which makes spinning impossible.
First it's easier than spinning and helps you get used to going all the way down. See how many times you can go up and down on the same leg. As for spinning on an outside edge, try and remember the same tips I gave for the scratch spin regarding shoulder positions. When you go down into the sitting position, bring the free leg in from the side as opposed to having it in front and plopping straight down. This helps to prevent traveling and gives the spin more speed.