On the tent or tarp end of the guy line I just make a lark's head with the rubber band through the grommet or loop on the tent, and then attache the guy line like I would directly to the tent or tarp. Or, on the stake end, I just loop the rubber band around the stake and then attache the guy line to the rubber band instead of the stake. If I want a stiffer shock aborber, I just push the rubber band through the grommet or guy-loop and . . .
I'll try and use a photo . . . In these photos the guy line is just a piece of cord with a bowline loop in the end making it easy to attache the guy to a grommet, or loop, or rubber band, however I am inclined at the time.
tarp guy 2.jpg Here is the softer shock aborber lark's head around the grommet.
tarp guy 1.jpg Here is the double stiffness shock aborber by just pushing the rubber band through the grommet and hooking the guy line through both half-loops of the rubber band.
In general I find the single loop is good for non-ridge-line guys and the double loop is good for a ridge-line guy of an ultralight tarp. On these slightly heavier cheap plastic tarps, like shown in these images, I use a full ridge-line and run a single loop rubber band to a prusik knot on the ridge-line to tension the tarps ridge-line.
Also, there are many times that I don't even use a guy-line, I just run the rubber band through the grommet/loop and then stake the rubber band to the ground without a guy.
I hope that is at least a little clear'ish. I can try again if need be.
Another thought, I've heard people express concern about rubber bands breaking. I've used this in some pretty fowel, stormy weather and never had a rubber band break. But, if the wind gust was enough to break the rubber band, it would alternatively pull the stake or tear the tent/tarp, so a broken rubber band might be the more desirable option.