Tarps mostly have convenient eyelets; I imagine a modern light tarp made of some kind of plastic. Often however, the eyelet can be gone for some reason or not in the right position for an attachment; maybe one would prefer to put a bridle to more than one point where there are no holes, or one could opt for attaching a cord to the centre to lift it so that water runs off.
Then I use a small object, as a pebble, inserted from one side and tie the string around it on the other side, so it acts like a button. No hole to let water through, and a secure fastening that will not be ripped out of the tarp as easily as a riveted eyelet.