There are many different ways to build this sort of structure with different knots. We've only used two knots and three types of lashings.
The equipment you need is:
Six poles (3.5+) in length
Rope - we used seven x six foot lengths and various long bits to lash it to the tree and the metal fence post
Four large wooden tent pegs
A swing (though you could do with having a pre built one, you could also use a seventh short log.
Step 1: What You're Building
A-Frames look like A's.
The top is joined by a Sheer Lashing and the bottom beam require two square lashings. Both lashings are started off with a simple clove hitch.
Both lashing are simple. I started off with demoing them, then let the scouts get in there. This is the first time the scouts in the photos have probably tied them. They're not perfect, but they held up and it's practice, practice, practice!
Step 2: A-Frames
Step 3: Bring the A-Frames Together
The top needs to be lashed together and we used a figure of eight lashing.
Step 4: Guy Lines
Step 5: Attaching the Swing
Enjoy your swing!