"Sit-ups
For regular sit-ups, the start position is with the back flat on a mat or
some soft surface, knees bent at a
90-degree angle with your feet flat
on the mat (back, buttocks and feet
are all on the same surface), and
fingers interlocked behind your head
throughout the exercise (Figure 7A).
Someone must hold the top of your
feet during the exercise, or you can
secure your feet under a fixed surface.
To complete a repetition, raise your
upper body forward by curling your
torso to the vertical position (back of
the neck forms a straight vertical line
with your hips, Figure 7B), and then
lower your body until your shoulder
blades make contact with the floor
(Figure 7A)."
From the Physical preparation guide for appointees regarding the APFT.