With apologies for the slight delay in update.
Once the first part of the mould has set (leave for at least 24hrs), we can press on. To do this, we flip the mould over, so that the wooden base is on the top. Gently remove the wooden base (it will come off will a little persuasion), in most cases the bulk of the plasticine will come off along with the wood. It doesn't matter however if it remains in the mould.
With the wooden base removed, take off the first two layers of lego bricks, this will bring you down to the level of the rubber which has now set. Having done this, remove any surplus plasticine which may still be stuck to the rubber or figure. Normally the bulk of the plasticine will just "peel off" but you might get the odd bit still stuck on. Use the sculpting tools to remove any residual.
In this photo, I've reached this stage, with the figure and mould "cleaned up" having removed any plasticine that was left. The mould box now needs to have the lego walls built back up ready to pour the second half.
As this is a "drop casting" mould, i.e. it will rely on Gravity to fill it, it will require to be "vented" to allow the air to flow out as the metal flows in. In most circumstances, the cutting of vents can be left until both halves of the mould are complete. Indeed for this mould I will be doing this and will illustrate this process in due course.
However as I've already highlighted, this mould has a slight "problem" namely the coat tails and turnbacks. This part of the figure will effectively be "buried" in one half of the mould. As such it will be quite difficult to cut vents to this part of the casting when the mould has been made. There's a real risk you could damage some of the detail in doing so. I have therefore very lightly glued a couple of plastic rods to the very highest point on the end of the coat tails. The rods are then laid on the outside edge of the mould. I will now pour the second half of the mould, and once set, will withdraw these rods, thus creating an air vent to this part of the figure (at least that's the theory!)
Before pouring the second half of the mould, I will again spray the surface and figure with "releasing agent". The rest of the process is exactly the same as previously detailed for the first half. You will need to mix up about 100g of rubber again.
Next time, finishing the mould.