The first step is to build a foundation which will decide the size of your grill and most importantly your concrete top. This foundation is made of wood, treated to be able to be used outside as it is.
These wooden boards and beams are cut to appropriate lengths and assembled. The target is to get four stable legs and a box where an isolator can be placed and the fire be made. I'm using standard concrete blocks for this. Be careful when making the size of this as it will be more difficult to cut the concrete blocks than the wooden boards (as I noticed).
I've used screws to assemble and hadn't planned on the angled supports but realised they would be needed as it was a bit unstable before they were added. At the top of the four legs I screwed four bolts into the wood, leaving them sticking up a bit. These are meant to be moulded into the concrete top.