Framing the structure
It is definitely better to pre-fabricate the frame, or at least parts of it, on the ground. That way you can use vices and power tools in your workshop. I always find that pieces of wood that I cut and join together in the treehouse are never quite square or the right size. It is much easier to work when you don't have to worry about falling out of a tree!
So long as the floor is big enough, you can make the frame any shape you like and then fasten it to the floor. I normally use brackets to screw the uprights of the frame to the floor. This is a strong joint but it will wobble about until you put your walls on. The walls add lots of bracing and will make the wobbliest of frames rigid when they are fitted. However, you may want to cut holes in the floor and attach the uprights of the frame directly to the floor supports. This creates a very strong and rigid frame before you have even added the walls.
Since the frame is built on a flat wooden surface, rules as to its construction are exactly the same as for any ground based wooden house. This lets you use a house building book to help with the design of your treehouse.