This is a super easy project. It converts the new fangled sling swing that is all that is available since the lawyers got through with it, to the kind of swing that I grew up with: a flat bottomed wide surface that didn't squeeze my hips and turn the boys vertical.
Step 1: The BOM
1) sling style swing that is all that came with the swingset
2) 1 deck staircase step from home depot. this was $10, and I only needed a piece of it. the advantage of this piece of lumber is it comes already rounded on one edge, and has a nice design pattern cut into it.
3) finishing stain and finish. This is optional as the stair is pressure treated, but It really makes it nice. look up my previous instructables for the the formula I use.
Step 2: WYSIWYG
So the build is really simple. you just cut the wood so that is a 2-3 inches longer than the swing and then on the the four corners cut out a rectangle so that the center part is left and can fit into the metal wire hooks for the doner sling swing. thats it! Its pretty stable and a whole lot more comfortable.