lta

Thanks so much! I stumbled onto his videos a few days back and they are great but I think I will take your advice and just put the laptop down!! 

Just want to say BRAVO!  Have no fear and  jump right in.  Even if you ruin it, you'll learn from it.  Biggest problem I run into teaching is that people are /afraid/ of failure.  I've killed countless plants learning how to grow and propagate them, and I've ruined many a painting and sculpture, mold, and casting.  But I learn best by DOING.  So jump right in and don't be afraid.  You got this!  Start simple, take small steps and if you need help, that's what we are here for!

Pick something really easy, like a piece of fruit - apple, orange,  banana -  or an existing cup or mug.  (Bear in mind that anything cast will end up smaller than the original.)  That will get you thinking of undercuts and how to section off.  

Thank you for the video, i think I may have watched this one before! It's a great video but never thought to apply the technique to what I am doing now, makes sense though. I'm just afraid to take the plunge because it could easily be a three-piece mold because of the undercuts and detail. But actually after watching this I may take a break and do something more straightforward and cast that first just to get my feet wet. Thanks for the imput!

There's no problem using a plasticine form to cast plaster molds from, it could be a bit tricky to block off areas for multi-part molds but some normal clay will probably remove pretty well when you come to pour the next section.
I did read some plasticine can mess with silicone curing but nothing about plaster.  My only advice is to get a soft natural hair brush and paint on a thin layer before you do the main pour, that's the only way I found to get rid of most/all air bubbles on the surface. Tried different surfactants and different soaking times and mixing times and they all seemed to trap bubbles on the surface no matter what.