We used two 96" (8-foot) 2" x 4"s for the long ends and two 34 5/8" 2" x 4"s for the short ends (takes into account the true dimensions of 2" x 4"s so we would end up with a frame that was exactly 36" wide to match the width of our plywood).
My husband cut the short end pieces with the circular saw because he's fast and I'm afraid to use it. I am, however, comfortable with the jigsaw and could have easily used it to cut any of the 2" x 4"s.
We added three more 34 5/8" horizontal cross braces to the rectangular frame. These add stability and double as roosts for the chickens!
We then squared up the frame and added diagonal cross braces.
Every connection was pre-drilled and screwed using two 2 1/2" galvanized deck screws. It is easier if you have two guns: one for drilling and one for screwing.