Enlist the help of several friends to make unrolling the tarp on the roof easier and safer.
Open up the tarp and extend it over the leaking area so that it extends several feet below the leak and all the way over the peak of the roof above the leak. If you don't run the tarp over the peak, water will simply run underneath the upper edge of the tarp and the roof will continue to leak.
Tie ropes into each of the grommets on the edge of the tarp, run the ropes down the surface of the roof and tie them to anything that is handy. You can run the ropes over the edge of the roof and tie them to trees or to rocks on the ground. You can also put nails into the fascia and tie the ropes to that. Be sure that the tarp is well secured.
Place flat, heavy objects on top of the tarp if your roof doesn't have a steep pitch and you are expecting high winds. Don't do this if there's any risk at all of the objects sliding off the roof.