Well, you tell me. Either way, you start by lying flat on your back, with knees bent, belly button to spine, hands not behind head.
Before we continue, a little background:
Sit-ups work your rectus abdominis (your six pack!) ... somewhat. They also recruit your deep hip flexor muscles, your quads (front of your thigh), your obliques, and a few smaller leg muscles.
Several years ago, at the dawn of "Six Pack Abs" programs and "Guts and Butts" classes, "we" decided that sit-ups are unsafe and ineffective, and that we should never do another one as long as we live. Well, we found out later that this really depends the individual. If you have a history of low back pain, are deconditioned (AKA out of shape), or are feeling acute low back pain during sit-ups, then sit-ups are not for you.
Otherwise, sit-up away! The benefits to doing a sit-up over a crunch are that you're simulating a functional activity that your body will have to perform multiple times every day for the rest of it's life, and that it is a good whole-body exercise (which is cool if you're short on time).
Follow these great instructions from ExRx.com, and see pictures below for a How To. Feel free to comment on the gaudy hot pink arm warmers. I am secure in my armwarmerhood.
So... if you're still reading then it's probably because you decided that you're not yet strong enough for push-ups or you have a kooky back. Fear not, you're still gonna feel something, and it'll make you stronger.
Ahem, the crunch: