We found this great how to make a shower curtain via eHow and have abridged the directions to give you a sense of it. If you want to go forward we definitely recommend that you read the full step by step.
1. Measure your shower opening and add 16 inches to the width and 8 inches to the length for the amount of fabric to buy.
2. If you're fabric has a pattern, you'll need to match it since most fabric comes 54 inches wide (and a tub will be closer to 72). Easiest is to cut one panel in half lengthwise and match up each half on either side of the uncut panel.
3. Sew the hem and measure and mark where you want the grommets to be.
4. Attach grommets using the directions on the grommet package (seriously).
And of course, the most fun part of this is picking out the fabric. Here's just a taste of what we'd be into: