Step 1: Fabric and measurements.
Go to the fabric store and pick a fabric you like. These curtains will be cut so the "grain" or warp is running top to bottom, so make sure the fabric bolt is at least as wide as each curtain you want to make. Buy enough yardage to accommodate all your curtains. I bought 4 yards of 48in wide fabric to cover 2 50in x 30 in windows and it worked out well.