OP! There are a couple of examples here of the post and ring beam approach.
It’s handy for sloping ground. I just dig the 4×4 posts about eighteen inches deep. I don’t use concrete to pack them in, just the material that came out of the hole. Once the first post is in and plumb, stretch a length of builders string along it in the direction you want to build and dig in successive posts against it. I used wooden spacers which I clamped then braced posts until I could nail the ring beam too.
Make a Pythagorean triple to determine the square to the corners and do the same on the opposing side.
http://www.petermclarenfineart.com/Blog/Studio-Build-Blog
Hopefully the pictures should still work in this link too
http://singletrackworld.com/forum/topic/diy-garden-pavilion-ideas