How do I prepare plaster casts for casting tin figurines?
You need to minimise bubbles in the plaster of Paris mould. I do this by mixing the plaster of Paris in a rather runny state and after pouring into the former for the plaster tapping it to release any bubbles that have formed. 
Plaster sets relatively quickly but even after setting is still wet and in thick sections takes a long time to dry adequately, which is why baking might be a good idea if you can't leave the mould to dry thoroughly.
The mould will need a riser channel, in addition to the channel you are going to pour the metal into, to let the metal flow properly through the mould (and so you can see that it is properly filled). It will also need breather channels to let the air out of awkward corners and re-entrants.
Finally after the metal is poured into the mould, if it can be done safely, the mould should be tapped to help air escape.


Something else to consider is keying the parts of the mould together so that they properly align. Normally I do this by making circular/conical depressions in the first part of the mould to be formed, so the second poured onto it forms keys with these features.