One great method is to do when molding a bio out of silicone is a brush up mold! There are plenty of videos and articles describing how to do it and what to use. Smooth On actually has a video too.
Smooth On's Rebound 25 is a silicone made primarily for brush up molds due to its thickness for vertical molds. It can also be thickened with thier Thic-Vex (I believe that's what it's called) to thickened later layers to fill undercuts. If not, mold max 30 can also be used, but fast cat is recommended to speed up the pot life (how long it takes to thicken up). It's not necessary, but it helps so you don't brush silicone for 45 minutes lol
For the shell, you can use a number of things (plasti paste II, fiberglass resin, epoxy, plaster bandages). Find whatever you are comfortable with! Punished Props also had a great video on brush up mold!