No reason to keep the instructions private. Here goes.
Stuff needed:
1. Go to Smooth-on.com and pick the TIN CURE silicone of your choice. Oomoo-30 is what I'm using, but that was local availability.
2. Get a can of mold release wherever you get the silicone
3. Find a FLAT bottomed container that will completely enclose your model, but only has about 1/2" extra around the sides. Plastic cups work great, don't worry about any writing on the bottom of the cup.
4. Get a small amount of non-hardening modeling clay
5. 2 measuring containers and one mixing container (plastic cups) and a flat stir stick (popsicle stick)
6. Rubber or nitrile gloves
Process:
1. Roll a bead of clay smaller diameter than the top of the boot. This will be a pour channel and will get cut off the castings. Make a circle of this and stick to the top (fattest) part of the boot.
2. Put the boot, clay side down, into the container. Make sure it is centered
3. Spray with mold release
4. Measure and mix up the silicone
5. Pour over your boot. Cover the boot by at least 1/2". Let cure overnight - DO NOT HEAT CURE THIS, you will destroy your precious real boot.
6. Remove the mold/boot from the container and work the boot out. You are now done with the real boot
7. Spray mold release into the newly created mold.
8. Measure and mix more silicone, fill the mold, let cure overnight
9. Heat cure per the directions
10. Remove your new boot, rinse and repeat.
Have fun. I store my stuff in the refrigerator, it seems to last longer than advertised this way.
Mold Making and Casting Materials by Smooth-On
Chart of all smooth-on 'rubbers' some are silicone and some are urethane. I think the silicones are better. I would not make the mold with anything stiffer than what I sent you. BTW that is Oomoo 30. Your casting can be with stiffer stuff. Personally I would get some Oomoo 30 and make the mold and then cast a boot and see if it works for you. If not you have the mold and you can cast a stiffer material. Buy trial sizes - tons of material for you.