One way that works for me is to completely "unsew" the seat upholstery and lay it out on a sheet, paper, or something else that has been trimmed to the width of the replacement fabric. That way you can come up with a good estimate as to the length that you will need. Be concerned with the pattern and be sure the pieces are laid out correctly. I always allow plenty of room for error and generally order at least ten percent more than I need.
We (my wife is a wiz with the sewing machine) then cut the new material to match the old patterns, adjusting as she deams necessary, and she sews it together using thread that is appropriate for the material. I find that the ladies at Joanne's Fabric are a great help. JoAnnes also has a huge selection of material and frequently offers 50% off coupons. Gar Williams