Make it tight to avoid losing too much dental stone down the side. You can also use something like the manila folder cardboard strips noted above. The dental stone will soak through, but not compromise the integrity of the dam or mold. Place the tiles on the blank. Mix up the dental stone.
Dental stone cures fast.
Don't waste any time. The amount and consistency you'll need is an art more than a science. For the tool's thickness I guess a good rule of thumb is to divide the diameter of your blank by 6. The consistency range is pretty wide. Both have worked great.