The process as I understand it is that you start with a lifecast/armature to sculpt on, then you make the mould from the sculpt, and then reinsert the lifecast/armature into the mould to make the core, so you're left with a cavity between the two. Typically this is for silicone masks, so the inside of the mask fits the person the lifecast was made from perfectly.
But if you've just got the mould and you want to make a core from that, I guess you could cover the inside of the mould with a thin layer of clay, and make a cast inside that to make a core slightly smaller than the mould.
best way to do it is to take some plastic wrap lay it in the mold than cut clay any kind will work but water clay is easiest and cut the clay in even thickness strips as thick as you need them for foam latex 1/4 inch is best.
after the clay is cut place it in the mold over the plastic wrap make sure you cover all of the mold. now this next part is very important. on areas of the mold where you did not sculpt like the shoulders or breast or even in the eyes and mouth area for example if it is like a superhero mask you must add keys.
to add keys what you need to do is cut circles in the clay you just added down to the stone. these hols or keys will prevent the core from flopping around in side the mold. depending on the size of the mold and area of the life cast that was not sculpted on will determan the amount of keys you make and the size of them.
ok now that you laid the plastic wrap and clay and cut your keys it is time to add another layer of plastic wrap. you dont have to it is optional but it will protect the clay so you can reuse it. once all of that is done clamp the mold shut like normal. once the mold is shut you can than pour your plaster or ultracal into the mold. you can do it sold plaster or add burlap for strength.
now if you want a lighter core you can make a fiberglass one also it is all the same steps except when you go to cast the core you have to do it one side at a time with the mold open than after each side is made fiberglass them together