Think of roof shingles. Start the debris layers at the bottom, and work your way to the peak, overlapping as you go. You will have to repeat this about 3 or 4 times, adding some more overlapping layers.
If rain is imminent, Make the shelter walls as steep as possible. The steeper, the more the water will follow the debris to the ground.
Evergreen material is your friend.
Don't forget a good layer for bedding to keep you off of the soaked ground.
If i ahd the choice, I'd take plastic or large poncho.
If you want to do it au-naturale , then go for it. when you think you have piled on enough debris, add some more.
Site selection. If the wind, is, say, out of the SouthEast, then find a large tree, and place your shelter close to it, on the Northwest side. it can help reduce wind blown rain.