Tarps can be used above ground for a hammock, or on the ground like a tent. It is quite easy to hang a hammock tarp. Hang your hammock before you hang a hammock tarp. Essentially, there are 3 main steps.
Fix the ridge-line
Some tarps have a reinforced ridge-line and/or, pre-attached guidelines. When a tarp has pre-attached guidelines, these lines can be used to attach to a tree at either end using a farrimond hitch. Tarps without ridge-line guidelines need a length of para-cord from tree to tree above the hammock. Typically, the height of the ridge-line is dictated by the height of your hammock. Your hammock should be hung with guidelines running from the tree to the hammock ends at roughly 30degrees. The hammock tarp should be roughly horizontal. Since your tarp should not touch your hammock, this sets the lowest position. The highest position is probably determined by how high you can reach, but remember, the larger the gap between hammock and tarp, the more weather can get in.
Fix the corners
All tarps should have either an adjustable toggle or a simple loop in each corner. If not, fix these before you leave or use the technique used to hang clothes to provide a loop. Imagine a straight line from the center of the tarp to the corner. Corner guidelines should continue this straight line from the centre of the tarp, to the corner and hen to the ground. The angle of the tarp from the ridge-line to the ground is determined by how far away the stakes are placed. Assuming the tarp is being used to keep wind and rain out, the angle between the two tarp surfaces should be around 90 degrees. A more acute angle and your hammock may touch the tarp. A more obtuse angle may mean too much draft can get in. On occasion it may be easier to tie to another tree rather than use a stake. The type of ground (rocky, sandy etc.) will determine what kind of stake to take.
Chances are that once the ridge-line and corners are fixed, the angles are off and/or parts of the tarp are loose and wrinkly. Assess where lines need to be adjusted or moved and tighten everything up so your tarp has the angles you want. Tighten the lines so all the lines have a little tension to keep it in place.