If you're wanting to do center tieout, you will need 2 pieces of reinforcement material(70d ripstop nylon works good) cut in a circle of about 3in diameter. Pin one on each side of the fabric where you aim to put your tieout. Stitch the 2 reinforcements to your tarp material, then place your grosgran tab in the center of the reinforement patch and either bartack it, sew back and forth over it a few times or even zig zag stitch it a few times. The hardest part is sewing the circular patches without them bunching up or your stitches being crooked. I have found that if you put a thin layer of silicone glue (GE Silicone II or similar) on the patches and let them dry overnight before stitching; it makes for a much cleaner look as well as being easier to sew.
Hope this helps