This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
The sit spin is done in the "shoot the duck" position, with the arms
extended in front of the body and the free leg in front and turned out
(as in ballet's rond de jambe). There are many variations on the entry
and body position, but here is one that works for many beginners.
Start the spin in the sit position. When stepping into the spin (on a
LFO edge), bend the knee deeply and lean forward trying to place your
chest on your thigh, only keeping your back straight and head up. This
is to get you to enter the spin in a lower than usual position, so
that you don't start spinning in a one foot spin and then have to
bring your leg around AND lower your body at the same time.
It has been suggested that you should try to look over your right
shoulder (left shoulder for CW spinners) and see the heel of your free
leg as you enter the spin. Get your balance in the spin before bring
the free leg around -- about 1/2 a revolution. Once in position, sweep
the free leg immediately around to the front. You want the right leg
and right arm go come around at the exact same time so timing is
critical. Place your arms in front as if you were water skiing.
Here are a few tips:
-- Practice shoot the ducks (skating forward and then lowering
yourself into a sitting position, arms and free leg extended in front)
in a straight line and try to reach past your toes. This helps you to
get into a nice low position and gives you more of a feel for where
your weight should be in a sitspin.
-- During the entry look out at a point beyond your hands and keep
your back straight.
-- Keep your butt down and shoulders back. It's common to feel the
need to keep your weight forward so as not to fall back. This will
only tip you onto your toepick!
-- Remember to turn out your free leg (the inside of your foot should
face the ceiling).