When we opened the pool for the summer, we discovered that twenty or so tiles had come loose over the winter. They are intact but need to be replaced. My husband has DIY experience with floor tiling so we were hoping this is something we could do ourselves. Could you please advise on the process and products we should use. I've attached some pictures. The tiles are above the water line on a slope. Some of the material under the tiles has been chipping off as well. I'm guessing we will need to build up a flat surface before we begin. Thoughts?