OK, first be aware that the 10 by 12 tarp will be a little shorter, too. This might work out better for you, because any edge that you have to cut will unravel over time if you don't treat it. Hopefully you can keep two and possibly three factory edges. I'd recommend that you cut any edges an inch longer then use contact cement to fold back and glue a 1" edge.
Where you need grommets you need to add support. Duct tape works but tends to peel off with use and leaves a gummy residue. If you can get a pack of tent patching material you can cut patches to size then peel and stick.
The next time you are shopping stop by the women's hair products area, and look for those elastic loops that the ladies use for their hair. Pick up a pack of the thick ones. Take one, push a loop thru your grommet, then pull the loop back thru the elastic band. Pull tight and you have a great stake point.
