Here is the process I use when creating a mould and mother shell...
After sculpting (this is the left thigh), I put a few layers of silicon on (usually 3 or 4). The first is a straight mix of A and B which gives a thin, smooth layer. The next layer I put some thi-vex in to thicken up the silicon. The following picture is after the second layer.
After the silicon is relatively smooth with no indentations that the mother shell might get locked into, I add some bumps in the silicon so it is easy to align the silicon in the mother before I make the cast. I then use the clay to divide the mould in half watching out for indentations that could cause the mother shell to lock on to the silicon. I add a couple of registration bumps in the clay so the two halves are easy to fit together in the right spot.
Next I create one half of the mother shell. I use burlap and plaster of paris because it works well and is cheap. You have to work fairly quickly before the plaster sets.
After the first half is set, I turn the mould over and remove the clay (it can be reused many times but you might have to clean bits of plaster off of the clay).
I then apply foil tape to the edge that the second half will go against. I find this works well because the plaster of paris doesn't stick to it but it conforms to the contours of the edge really well - even if it isn't sticking well. I fold it over the outside edge which helps keep its shape even more.
Then it is time for the second half of the mother shell. I usually cut out sufficient burlap before mixing the plaster of paris. Here in Canada you can get big rolls of burlap from lots of stores during the fall (for protecting bushes from snow) but it may be harder to find in the spring or summer. I ran out and needed some more and ended up buying a long burlap table runner from Michaels. With a discount coupon the price wasn't too bad.
Now I wait until the morning for the mother shell to fully set and dry out before separating the halves.