I've listed 25 different tarp shelter designs to help you get started. Each configuration has its pros and cons and there isn't really a perfect design for all occasions. You'll have to chose the righ(Try Design)
If you need to pitch a tarp or tighten a fly, you can make an impromptu tarp clip from a split twig, a matchstick and a bit of string. Works best if the clamping part is long enough to go over the folded edge of the tarp.