Floyd;
I was a complete novice when I decided to sew my own TR3 seats. I used marine grade UV-resistant vinyl in 41 mil thickness - quite a bit thicker than what you buy off the shelf from the big three. I didn't use leather only because I wanted to avoid any problems when matching colors (assuming seat fronts in leather and the rest in vinyl).
I disassembled the old seats that were on the car, carefully taking apart seams stitch by stitch. I then traced each separate piece onto paper, scanned the paper, then imported the scans into Adobe Illustrator and used them to create vector-based patterns. I then had Illustrator break up each piece into letter size chunks with a quarter inch light grid in the background so I could verify sizing after printing the patterns. I went through all this in order to be able to publish my seat patterns for use by other members of the forum, but you certainly don't need to go through this type of pain and agony - just trace each piece onto your new vinyl and cut them out. It is useful to make notes for whoever will be sewing things together as to special seam types (pictures help here).
I bought a heavy duty machine, nylon matching thread, and dove right in. You can see some of the results here:
https://xjguy.com/triumph/2009feb2/800x600/00011.jpg
FYI, I sewed my own piping - bought some thick twine and sewed it into a 1.5 to 2 inch wide strip of vinyl.
You might want to take a look at the document I published about the process - it has pictures, patterns, and describes everything I did. My TR3 patterns won't be of use to you, but the pics might. If you need it, email me offline and I'll email it back to you.