This is similar to what we did at the Camporee, except I had to drape it over the tent & stake the tarp out because we didn't have the extra two trees. It still worked well & we were dry. We had a nice vestibule at the door.
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 right one depending on your situation or you could just try them all out to test your bushcraft tarp setup skills. Click the image for more info or go to http://rollingfox.com/how-to-make-a-tarp-tent-with-designs
various shelters using a tarp - weve contemplated having a class for older kids at an event where we teach them about basic medieval tents and then give them a tarp and rope and let them construct their own place - what teen doesnt want their own room even when SCA camping?