There are plenty of harnesses stretched out for all to see here:
http://www.thelongandshortofit.com/harnavest.htmIf you already have fabric, I would take the cheapest stuff and imitate what you see on that website and place it on your dog, adjust from there.
Do NOT use the D rings from the fabric store. They will open up and you'll be standing there with just a leash in your hands and your dog will be gone.
I can't seem to find the pricing on this website but I know Adele from her coming to our meet ups. If you buy a vest from her (her D rings are excellent) you will already have a custom fit for your dog and you can use the harness as a pattern for future harnesses. It's Canadian funds so you get a break on the dollar too. You can look at it as the last harness you'll have to buy, and it is ready to go until you put one together yourself.
http://dawgydesigns.shawwebspace.ca/pag ... harnesses/Good luck. Hope to see photos of your finished product, modeled by a handsome little fella.