Ok here is the process I use to shrink a head where the mold shrinks. The first way is the Smooth On process using their NOVOCS solvent and Mold Star 16 silicone rubber, the solvent thins the silicone and evaporates shrinking the mold and they have basic instructions on their website. Buying the material will run around $70 and unless you have a lot to shrink that seems expensive. The method I use s based on the same process but doesn't cost as much.
The silicone rubber I use is Amazing brand from Hobby Lobby and if you use their 40% off coupon it will cost about $13 and you can make 3 molds. Normally I don't use it for molding heads because it doesn't degas itself well and there can be air bubble issues, it's ok for a pour over mold but not as good for a dip into mold like for molding a head.
Now for the steps in shrinking, these are general guidelines. I use Naptha as the solvent and the thinner silicone mix doesn't have the air bubble issues that I mentioned. To shrink a head from 9" size to Mego size I mix 3oz of silicone, 2 scoops of catalyst and a generous 1oz of Naptha - no more than 1.5oz, don't go over a 2 to 1 ratio. The more Naptha the more shrinking but the mold will also be weaker. It usually take about 4-5 oz of material for a 9" head mold but you check beforehand with some water and a measuring cup in the container you are using for the mold and adjust things accordingly. Let everything dry before pouring the mold. Mix the silicone and catalyst thoroughly and then add the solvent. It will take maybe 5 - 10 minuets to mix the solvent in thoroughly and there is an even texture. The cure time is about 4 hours, you'll know it's done because the mold will feel firm. After removing the head use a rubber band, not to tight, to keep the mold together. This is an evaporation process and there will be a tendency where you cut it to dry quicker because of more exposed surface and to open up at that spot so the band keeping things together helps for an even shrink. Also this process doesn't seem to work as well for bigger molds, 1/6 scale heads, some shrink well and some don't.
Molds should be as shrunk as it's going to get in 5-7 days,
One thing to keep in mine is that Naptha has fumes so use in a well ventilated area.
Also, I wouldn't use this process with Oomoo as that is a weaker silicone rubber to start with.
Use 5oz paper cups for molds
Given a choice and if I were going to shrink enough to make buying the Smooth On stuff worth it I would buy the Smooth On but for an occasional couple of heads this other way works and I don't have to worry about things going bad sitting around