How do use plastelina to make a mold?
The plastelina is used in the process of the mold making but is not the mold. I found a very clear tutorial explaining the use of plastelina in making a silicone mold. Here it is: http://aldax.com.au/2partsiliconemould.htm
If you are about to make a piece mold, the plasticine would be used as low walls perpendicular to the surface of your sculpture. Then you use plaster to create the first piece, and later remove the plasticine wall. For the next piece, the plaster cross section wall can be given a coat of vaseline and you could use plasticine again if you are going to make a multiple piece mold.
I would prefer to visualize in reverse, that is from what my final sculpture will be made. If its going to be plaster or concrete or any other hard material, then either I make a piece mold to escape the undercuts, or I will have to plan to break my mold to retrieve the cast. If its the second option then I would color my first coat of plaster with a bit of red oxide to warn me when am too close to the cast surface as I break the mold.
But a word of warning if you plan to cast in rubbers or plastics, because in all probability the plasticina would be sulphurated and would inhibit the setting of some rubbers or plastics.
And finally if your plan is to cast flexible material then you may not need to create a complex piece mold or even break it.