At a fraction the cost of that purchased in stores

You'll need:

Mix small amounts as a litle goes a long way. In a jar (that you don't need again) mix the silicone and the mineral spirits in a ratio of about 1 to 1 or slightly less silicone than mineral spirits. Some people use 1 silicone to 2 spirits to make sure it flows well into the seam. You might want to experiment in a corner of your gear first. A couple of ounces of finished product should do for your first try. Start stiring. You'll know when your finished when it's no longer lumpy and has a nice smooth consistency like heavy cream. If you've ever purchased some, you'll know what it should flow like. Make sure it will flow nicely so that it will get into the seam and not just sit on top. Work it in well with the brush.
I seal my equipment in the back yard. The tent is setup so the floor and sides are nice and taut. The fly is draped over the picnic table and is pulled taut by a helper.
Don't forget to have a clothes-line ready for the fly. The tent can be left upside down with a couple of guy lines to keep it that way. Let dry overnight.
A neat trick is to make a large cross-thatch on the inside of the floor to stop those darn Thermorests from sliding around. Another is to cross-thatch the top of the Thermorest so you and the sleeping bag don't slde around.

DON'T forget the Talcum Powder

Rub a little talcum powder on the DRY seam before putting it away. If not, all seam sealants including commercial will stick to itself. They all dry in a few hours but take a few days to set.