First measure the size of your old cushion from seam to seam.. Does it have sides? or is it like a stuffed pillow? If it has side pieces then you will also need to add the 3.5" to Length and width. If you dont have an old one to cover then buy your stuffing and stuff it. If your going to make your own and stuff it, then measure length and width needed to fit seat. Add 3.5" to compensate for drop caused by boxing the corners and seam allowances..
Cut two pieces. Now you could just use the exact measurements, 1" seam allowance and just sew it up without boxing corners, leaving about 8" to stuff it, turn right side out, and sew the opening closed. I like the look of the boxed corners.So here are the instructions:
1. Place right sides together and pin.
2. starting on long side of back of cushion about a foot from the corner, sewing it around. If your stuffing,
leave about 8" opening. If your stuffing your old cushion inside then leave about 15" opening.
3. Now to box the corners:
Flatten corners so that side seams lay on top of each other. The beige fabric is my underside, drop cloth fabric. the blue is the top. Pin.
Now measure down from corner 2.5" on each side and mark.
Now do the same thing to other side.
Now lay your ruler across from mark to mark and draw a line.
Sew across this line. Trim off excess corner leaving 1/2" seam allowance.
Turn right side out and poke out corners.Once you get your old cushion in or get it stuffed, sew the opening shut.
This is a great beginner project. especially if you used all drop cloth since it is so cheap.