When the sculpt is done and ready to be casted you need to consider how many parts you need to make the mould in, to be able to separate it in the end. Plaster is a firm material and avoiding undercuts is therefor essential, or else the mould will be impossible to take apart. Usually with a head a two part mould is enough, one piece at the front and one piece at the back, with the joint just on top of the head and ears.
You need either a clay wall to make the dividing line, or you can use brass fencing, as I have chosen to do. I prefer this since it is less likely to damage the sculpt, and I find it much better to work with if you want a neat result.
The brass is cut into small wedges that is then carefully pressed into the sculpt on dividing line. The wedges should overlap each other and this way create a solid wall.
In the back I secure the brass with Gaffa tape, to make sure it stays together, I also use some tape in the front on the bits that are a bit unstable.
Doing the walling up with the sculpt standing up, it is now time to lay it down for the moulding. As when moulding the beak, I use a plastic covered pillow, laying it down very carefully to avoid damaging the sculpt in the back. I then secure the wall underneath with some clay and polystyreene to make it able to take the weight of the plaster.
I then make a clay wall around the neck, to avoid the plaster from running down the edge and on to the plaster head cast.
To make sure the two parts fits together I mark out some joints with balls of clay around the edge, this way it will always grip to the same points when being put together.
I then cover the front in the same way that I did with the beak on character number 1, using a bit more plaster this time. So I need to be prepared to mix more than one go of plaster, using two different bowls, since the remainings of the first mix will ruin the second mix, and you don't have time to clean it while moulding.
Once it is done, you just have to wait until the plaster is firm enough to turn over, about 15-30 min. Then you can turn it over and remove the wall carefully. When the wall is gone it is a good idea to fill in the damage from the brass fencing on the sculpt, where the brass has been pushed in, to make it neat. Then you can start with the second half of the mould.
Remember that plaster sticks to plaster and you do not want this to happen! To avoid this I create a release agent with grey pottery clay mixed with water, the brushing it on all of the visible plaster, making sure to cover all of it. I then make some wedges of clay to make it easier to part once it is done, I place a few around the edge. Then it is time for the second half. While the plaster was setting on the first half I took the opportunity to clean out the bowls and make everything ready for the second half.
Repeating the process with the plaster on the first half, and then it is just to wait, at leats until the coming day to take it apart, since it will crack if not properly set. A good way to check if the plaster is set is to knock on it, it should make a ringing bright noise, instead of a damp soft noise, then you know it is properly set.