this one is not so much a step by step as different BBQ's will have different frames, but the procedure should be universal, and I wasn't planning an instructable when I started this project.
The first step will be to procure and old propane BBQ, if you don't have one in the backyard, try Craigslist or freecycle, I had two. one I converted a couple of years ago and it holds leftover yard sale items till next time. This cart I am using as a paint cart for mural painting.
you will need some scrap wood or metal if that's what you have. I used plywood, OSB, and some old 1X4's
you will also need an assortment of screws, both wood and metal. I reused some from the BBQ itself.
I also reenforced the back rail with a couple of metal L brackets. (behind the black tool holder)
next, remove the aluminum shell and all burner parts and brackets not needed to hold the frame together, you may want to attach the frame post to one another with wood before removing the shell, as it will be pretty floppy when it is removed.
I used a grinder to cut the bolts holding the shell on as they were to rusted to remove with a screwdriver.
I next used some scrap OSB board to tie the two sides together, then using some scrap wood from the BBQ, tied crosswise between the OSB, I just measured and cut as I went, going with the dimensions of the scrap available to me.
I had a partial sheet of plywood that was bowed, and I cut it to 25X55 but you will need to cut it to whatever your use will be. I mounted it with the bow up and forced it down with screws, into the cross support and the metal outriggers on the BBQ frame.