I was having trouble laying a large slab over a corrugated cardboard mould because the mould is so light that it moves with the slightest push. So I used the sandwich technique: thin board first, then clay slab on top, then a thin cloth smoothed over the clay slab, then mould upside-down on top of clay, then thin board on top of mould. Hold the sandwich together with one hand on each side, then lift and turn over. For large slabs, rest the sandwich on your head and change your hand position so it is easy to put the sandwich down. When you remove the top board the slab (with the cloth underneath it) will be laying over the mould waiting for you to push it gently down inside. The cloth keeps the slab from sticking as it slides down into the mould and it is easy to peel it off when the slab has dried to a firm leather hard.
Sandwich
I was having trouble laying a large slab over a corrugated cardboard mould because the mould is so light that it moves with the slightest push. So I used the sandwich technique: thin board first, then clay slab on top, then a thin cloth smoothed over the clay slab, then mould upside-down on top of clay, then thin board on top of mould. Hold the sandwich together with one hand on each side, then lift and turn over. For large slabs, rest the sandwich on your head and change your hand position so it is easy to put the sandwich down. When you remove the top board the slab (with the cloth underneath it) will be laying over the mould waiting for you to push it gently down inside. The cloth keeps the slab from sticking as it slides down into the mould and it is easy to peel it off when the slab has dried to a firm leather hard.