Lying on your back, put your knees up in the air so that your thighs are at a right angle to your torso, with your knees bent. If you like you can rest your feet on something, like a chair. Put your hands either behind your head or gently touching the sides of your head.
Now, slowly raise your shoulders off the ground and try to touch your breastbone to your pelvis, breathing out as you go. If you succeed in touching your breastbone to your pelvis, see a doctor immediately.
Although the actual movement will be very small (your upper torso should move through less than 30 degrees) you should try to go as high as possible. Only your spine should bend, your hips should not move. If the hips move, you are exercising the psoas.
Do these fairly slowly to avoid using momentum to help.
You can increase the difficulty of the exercise by extending your hands out behind your head instead of keeping them at the side. Make sure you don't jerk your hands forward to help with the crunch, keep them still.