The next step is to make timber wedges, so that the shower water runs down into the plug. Measure the shower base, diagonally across from corner to corner, then halve this figure, e.g. 1200mm will be 600mm. This is the length of the four wedges you need to cut.
Measure and mark the required length on your timber. Now clamp the timber to your sawhorse and cut the four pieces to length. Measure the depth of your shower base and mark that height at one end of your timber. Then measure and mark off half that height at the other end. This will give a good angle for the run-off. Use another piece of timber to draw a diagonal line between the two marks.
Put some powder mastic in a bucket and add a little water. Mix it until it's the consistency of toothpaste. Remove the wedges from the shower base and use a notch trowel to spread an even layer of mastic across all of the shower base. The mastic gives the screed something to stick to.