So the first thing I had to do was put the posts in the ground. Because I was building around a tree trunk and not in the crook of a tree or between two trees, I had to put posts in the ground to support the base.
I measured the places for the posts, dug the holes, then poured in the cement; checking and double-checking that they were square and positioned properly. It took a day or two for the cement to set before I could put up the frame for the tree house floor.
Once I could put up the frame for the floor, I measured and cut and put up the frame, being careful to give adequate room around the tree so it would have room to grow. This made the frame very sturdy.