For your viewing pleasure...

Being basically a box this is not the most exciting two-part mould you could ever make but it shows the basic principles.
Here's the model temporarily based on a sheet of plasticard, with a figure for scale.

internal wall added to make the resulting mould easier to remove casts from, and plasticine added to block the door and window and also all around the bottom edge to prevent leaks. The inside of the building will form one part of the two part mould, this is to make any resulting mould lines inside and below the roof line. Purely cosmetic really.

Balls are added to the inside plasticine, to help the finished mould sit neatly together.

It's then partially filled with rubber, degassed in the vacuum chamber and then topped up to the roof supports.

Once cured, a couple of cuts are made in the top, which like the balls above will help hold everything in place during casting. Then a box is added around the outside.

First a few coats of wax are srayed over the exposed rubber, to stop the rubbers fusing together, then a small amount of rubber is added over the model which is then degassed and topped up. This has been degassed a second time and the photo shows it before it's settled.

Next up... taking the master out and test casting.