with the Real Scale Ring out now, it's a lot easier and cost effective to just buy one. But, if you must make your own:
Make a base out of wood. This would be either two or four sides, depending if you want weapons underneath, and a mat. You can use just about any thickness of wood for these. This is also when you determine how tall you want your ring to be.
Take four wooden dowels and paint them to whatever color you want your ring posts (MUCH easier to paint them before putting them in than after). Next you need to add the dowel rods to your ring. Drill a hole in each corner the size of the dowels, then screw them in.
Add in 3 small J Hooks to each dowel. These should be at the same height on each rod. These will be the turnbuckles.
Next, you need to put foamies over the wood on your ring. Without these, your ring will be very hard and could damage figures, not to mention extremely loud.
Create a mat out of fabric for your ring, and put this mat over the foamies, hot glueing the mat at all four sides of the ring.
Next you need to add your ropes. You can either use Jakks ring ropes or elastic cords that you can get at a craft store.
Only other things left to do are cosmetic things. Add a hand made ring skirt and possibly turnbuckle covers and you're done.