Step 1: Layout of the main part of the cap
1. Measure around your head where you want the cap's bottom to be. Write this down somewhere you can get to it later because if you ever want to make more hats you'll appreciate not having to remeasure. For my hats I normally create one strip of fabric 23" long by 5.5" tall (I wear a size Small-Medium hat normally).
2. Using a ruler and a marker or pencil mark your cutting lines on the fabric
3. Cut out the strip(s) of fabric (in my case I made 3 pieces: two 10"x5.5" and one 4"x5.5" but normally I would only make one 23"x5.5" strip)
Step 2: Sew the top band together
If you're following what I did and using three pieces of fabric you'll get what's in the photos below, otherwise you'll have only one stitched area that will be the back of your hat.
Step 3: Finish the bottom edge of the hat
I fold and pin approximately 1" of the bottom of the band of fabric and then sew it twice; once around the upper part of the fold and once around the lower part of the fold. This gives a nice double stitch that looks good.