Start by laying on your back with a weight you feel comfortable holding overhead. The position of your legs is more or less negligible here, but you should not anchor your feet.
Press the Weight overhead, elbows locked out. Brace your midline and press the weight up as you perform a full range of motion sit-up. At the top the weight should be locked out overhead. Sit back down in a controlled manner without bending your arms.
Make sure not to turn this into a crunching type movement, you should have an upright posture at the top.