Finish your seat pan, your foam, and any attachment mechanisms before you think of sewing. Then get yourself some clear polypro dropsheet material (clear is nicer than newspaper or cardboard since you can see through it).
1)Take a sharpie to your foam and draw out where you want/need your seams to be.
2)Lay the clear plastic on the seat, hold it in place somehow (loops of tape, some spray adhesive, whatever) and trace the pattern of the seams on to the plastic.
3)Take the plastic off the seat and lay it flat on a table and true it up. When I say true, make lines that should be straight straight, curves that should be curves even, and make things symmetrical.
4)Give yourself alignment marks along the seams that you transfer to the various patterns. This way when you sew 2 pieces together they are lined up along the length of the seam and you don't wander, which causes problems.
5)Add your seam allowances to your patterns. This is how far to the waste side of the needle you want your fabric to be. For most things I use a 5/8" seam allowance, and trim it down later. 5/8 allows for a wide or narrow french/single reinforced seam should you want one.
6)Sew it up, being extremely careful to keep your seam allowance consistent and proper the whole way through, or you'll create some panels smaller and some larger, making your seams wavy and your cover alternatingly tight and sloppy.
Tips :
Use a long stitch. You may think that a tight stitch is stronger, but it is not. A tight stitch length puts more holes in the vinyl/leather, perforating it like a tear off piece of paper.
Go slow. If you can only manage to align the materials and spin the machine by hand then so be it, go at that speed. Focus on how exact you're being and not how fast you're going
Use a walking foot machine if you can. This is by and large the #1 thing you can do to make upholstery easier.
If you have any questions ask away, and be specific, I'll try to help the best I can. I sew a lot of auto upholstery for my business.
