Lie on your back, and open your legs into a diamond shape (aka butterfly legs) with the soles of your feet pressed together and knees out wide. Extend the arms overhead.
Inhale to curl the torso up, and tap the floor in front of your feet to stretch your glutes a bit.
Slowly lower back to the starting position.
This counts as one rep.
Do as many diamond sit-ups as you can in 30 seconds.