When the plaster is set, it is time to take the two pieces apart, which I have always found a bit scary, since you dont want the mould to crack, be stuck, or find damages on the inside of the mould. But the more you work with this kind of moulds the better they become, experience is the key.
Starting with clearing the edge from unnecessary plaster with a surform, until you can see the clay wedges that was put on the dividing line. Then, the best way is to soak the whole mould in water, since this one is very big, I use the bathtub to do this. This will soften the clay inside and release the two parts from each other. Leaving it in the water until you can see a 2mm gap on the dividing line, this might take about an hour. Then you can start to take it apart.
Be very careful taking it apart, opening it millimeters at a time all around the edge. This mould opened very easily, since I had been very careful when brushing on the clay slip release, this made my life so much easier when demoulding. It would be an absolute nightmare not beeing able to get the mould open.
Then it is just to clean out the mould with water, a dishbrush and a toothbrush. Once it is clean, leave it to dry, carefully tied together, since the plaster deforms a bit when drying, so it is essential to let it dry put together.