I've made a nylon tarp and hammock and repaired my pack once. I sealed the fabric edges with heat from a flame (don't let it catch on fire--easy to do! lol), and then sewed a patch of similar material which extended about an inch past the rip in all directions. Put a rolled hem on the edges of the patch, put in place with glue stick, sew close to edges of patch, then put a line of stitches about 1/4 inch on both sides of the rip. I'd do this patch on the outside so that the rip edges are hidden and don't grab onto things, causing a new rip eventually.
Edit: As Javan mentioned, use quality polyester thread, such as Gutterman. The edges of my rip were badly frayed, calling for fairly intensive patching work to actually replace lost material.