For seat you would basically get either a square or rectangle fabric. First hem right round or over-lock for neatness. you can forgo this if you like as it wouldn't show. If you have hemmed the seat fabric then tuck it into the seat firmly with the wrong side facing up. Pin the corners and draw a line for convenience. This would be your sewing line. Now push your hand in the back of the seat and pin if you can or at least make a mark with a sewing pencil. Sew all four corners. Fit it back into the seat with the right side up. Check if it is nice and tight. If it's loose all you have to do is make another stitch. Once you are happy with the fit, feel free to cut off the excess fabric and cord it, or over-lock it.