Add about 1 inch to the front and back edges of each pattern piece for the seams where you'll join the sections together. You'll need to know the width available to determine the yardage needed. This exercise will help you determine how much material to buy and which width gives you the best value and least waste. Just draw in the extra inches on the material. You don't actually need to make the paper pattern pieces any larger.
Dark colors can be too hot to sit on.
To sew your cover, use UV resistant thread and choose something for edging around the cutouts. Herculon or Stamoid work well, but you can also use the same material as the cover itself. Dacron line or equivalent. The drawstring basically follows the outline of the inflatable and needs to have enough excess to allow for installation and tying to the transom. Sewing Chalk or pencil for marking the material.