First, we cut out 18" squares.
Then we sewed two opposite edges of the napkin at a time.
Then we folded the same edge over a 1/2" and ironed it down. We did this to two opposite edges of each napkin, and then sewed down the folded seam of each side.
The finished product was a 16 1/2" square napkin, which turned out to be a good size.
After the shower, the napkins became a gift for the bride.
After making these napkins, I found this great tip at Marthastewart.com for quickly ironing even hems. Martha is ingenious, and I'm pretty sure she doesn't have to mess with an iron that shuts itself off every 5 minutes either.