I've never done it myself, but was considering it before + cake shop lady explained it to me. You need a cake board for each cake. The bottom, bigger cake goes on a board a bit bigger than the cake iyswim. The top cake goes on a board exactly the same size as the cake that will be sitting on it. You need some dowelling, which is sold specially for the job I believe. You get 3 lengths of dowelling, which you stick into the bottom cake, then mark with a pencil where they poke out the top of the cake iyswim, then take them out, and break/saw them off at that point.
You can then stick them back in the cake, and they will be the same height as the cake. The second cake then sits on its board, which in turn is supported by the three dowelling stick thingies, so they take the weight of the cake rather than the cake below, which would result in subsidence!
You then ice the whole thing. Does that make sense?
I'm sure you can find online guides, probably on youtube. Oh another tip she gave me - the big cake board, the one on the bottom, will be much easier to pick up if you glue a smaller one underneath it, so you can get your fingers underneath.