Plasticine is soft clay made of plastic so when you poor the plaster in there, it will come off very easily afterwards.
Make a flat piece with a rolling pin. Make it about 0,5cm or 0,2inch thick
Make some plaster ready. Mostly it is made with 1 part of water and 3 parts of plaster but it depends on the kind of plaster you use.
I always go with the pyramid method: you poor the plaster into the water at the same place and a pyramid will form. When the pyramid doesn't suck any water anymore, it's enough plaster and you can start mixing it with the water.
If you like that the plaster will set quickly, you can use lukewarm water or add a pinch of salt.
Poor the plaster into the plasticine mould.
You can also poor liquid (porcelain) clay in there and fire the stamp.
Let it set for a while and take the plasticine away. Cut away the rough edges with a kitchen knife.
Now you can stamp your heart out in any modelling material. :)