I have read articles on the net and in books on how to make a tile tool. I have tried a variety of methods and they all seem to do well:

http://www.johnstelescopes.com/HTML/TileTool.htm
http://www.kupercontrols.com/tiletool.htm
http://www.stellafane.com/atm/atm_grind/atm_plaster_disk.htm
http://www.atm-workshop.com/tile-tool.html
http://bobmay.astronomy.net/mirrormaking/mkplaster.htm

I have settled into the following method for creating a tile tool. This seems to work for me. Please feel free to provide comments and/or feedback.Please note, that I don't take any credit on the following procedures. This is simply a rehashing of a combinations that I have learned from the links above.

	Place a trash bag or something similar to protect the work surface underneath. Place the mirror face up on the trash bag.
	Wet the top of the mirror. This will allow the foil to stick to the mirror.
	Place some aluminum foil on top of the mirror and wrap the foil over the edge. Do NOT wrap the foil underneath the mirror! This will make the mirror be unbalanced and could cause it to tilt when pouring which is bad.
	I used a manila folder cut into 4" high strips and taped them together. This is used for the dam to prevent the plaster to pour over the side. This works very well.
	I also use a pipe clamp. This helps keep the plaster from running around the mirror (usually not a problem if you mix the plaster/hydrostone properly). I place the top of the pipe clamp to about even with the top of the mirror. Make it snug, DON'T OVER-TIGHTEN!!
	I use Hydrostone to make my tools. It is water resistent and gets rock hard quickly. You use 32 parts water to 100 parts Hydrostone by weight. For example, for a 6" mirror tool 1" thick, I use about 1.5 lbs of Hydrostone to about .5 lbs of water. Hydrostone is available in 50 lb bags which fill a 5 gallon bucket nicely. Make sure you get a lid that will seal to prevent water in the air from getting into the bucket.
	I recommend going outside to mix. It gets messy. Also, Hydrostone is an irritant. Don't breath it in and protect your eyes.
	Mix the Hydrostone slowly and evenly into the water. Stir well.
	The texture should be pretty smooth once you are done, similar to pancake batter (sometimes I have made it a little runnier and it still works). Make sure there are not any lumps.
	Pour the Hydrostone mix into the mold.
	After about 10-20 minutes, it should be ready. You can tell by looking at the surface of the mixture. If it is still reflective, it needs to cure a little more. Remove the dam and slide the tool off of the mirror once it is dry to the touch. If the tool will not budge, turn the mirror over so that it is on top. Slide a rounded metal object (rounded putty knife would work) between the tool and mirror and slowly break the seal.
	There will be an "anti-bevel" from the bevel of your mirror. Use a metal straight edge to bevel the tool on the top edge.
	Put the tool on a cooling rack overnight to cure. Next, we will seal the tool with epoxy on the back and sides.

Please contact me if you have any questions or comments.