1. Lie down on a bench with a barbell resting on your chest. Position your legs so they are secure on the extension of the abdominal bench.
2. While inhaling, tighten your abdominals and glutes. Simultaneously curl your torso as you do when performing a sit-up and press the barbell to an overhead position while exhaling. 
3. Lower your upper body back down to the starting position while bringing the barbell back down to your torso. Remember to breathe in while lowering the body.

Caution: It is best to have a spotter while performing this exercise so that it is easier to get the barbell and also get rid of it. Also, always start with an empty bar and do not use jerking motions at any time.

Reference: http://www.bodybuilding.com/exercises/detail/view/name/press-sit-up

The diagram shows the most used muscles on this exercise