I cut a strip of heavy poster board to make ring to go around the mirror. I taped this around the mirror which was covered with plastic wrap and and then poured the plaster. after curing I then wraped the tool with the same cardboard and then mixed my epoxy and layed my tiles onto the tool, and then put the mirror on top inorder to help form the tiles to the curve of the mirror.
By the way it does not seem, from reading other replies to this question, that others are using the sheets of tile where the tiles are hooked together in a preformed patern. This is an excellent way to do it. You simply lay your mirror on the sheet of tile and draw your circle then cut the tiles. In this way you can lay the whole sheet on the epoxy coverd plaster tool.
You allow the epoxy to cure but not harden before removing the mirror. Here you want to make sure you have several layers of plastic between the tool surface and the mirror so that you can lift the mirror off using the upper layers of wrap as a sling. I failed to do this and ended up having to slide the mirror off the one layer and that distured some of the tiles. If you use several layers this should not be a problem. once the mirror is off then you simply peel the last layer(s) off the tool.
Obviously, the surface will not be perfect but it should be close so that you can begin with the finer grits. Another point that we discovered by accident is that you want the epoxy t fill the spaces between the tiles. This does several things. first I can't imagine losing a tile that way. Second it all but eliminates the possibility of stray grit hanging around to scrats you work. I had no such problem with the tool which was made as recomended and several such events with the otherone in spite of efforts to avoid it. Thirdly, the epoxy will fill any low spots which gives you an indication of how flush your tool is. You can work with the coarser grit until most of the tile surfaces are revealed and then begin working down through the grits at that point.