We had an automotive upholstery business for many years. One of the simplier learning projects is a straight pickup seat, perferrably one that only needs the seating section and not the banding replaced.
Remove the covering and mark joining seams with a magic marker in several areas to show how the sections are joined together.
Now, using a cutting tool, rip the seams apart. (My wife perferred a single-edge razor blade...and could have those seats apart in the blink of an eye..
Use those pieces as patterns to cut the new pieces...marking the joining marks on the new pieces.
Match the marks and sew together. If you were able to reuse the banding/edging that the seat sections were sewn to, match the marks and reattach it to the new seat section.
By taking the seat apart you can learn how it was sewn together.
You might want to check out <www.carrscorner.com> Jack and the others are great about answering questions and can probably refer you to other sites.
Another suggestion....to become accustomed to your machine...before tackling an actual seat you might want to try some things like a tool wrap bag of heavy canvas or other material, a shift boot, etc.