There are several good methods of tarp tensioners, but my favorite is the headchange4u method. I think it's the least costly and the easiest to make. I have shock cord laying around at all times. The last set I mad was from 3/16" shock cord (I had it on hand), which is too big. It really turns the tie out into a stake launcher if I'm not very careful. When I get some smaller shock cord, I'm going to change them out.
Sometimes I loop it/ double it like in the post below. Others I have just tied clove hitches on each end of the shock cord.