You should first and foremost follow the instructions for your unit in this regard We would always recommend using a suitable waterproof sealant to seal your shower during and after installation to ensure you can fully soak every inch of the shower worry free. Ensure that the shower is completely dry before you proceed.
1: Apply a bead of sealant either side of the central tower vertically and between where any two panels touch and simply wipe away an excess the presses out .
2: Silicone horizontally between the tray/base and the back panels (AVOID the little water gullies/channels that allow water to return to the tub)
3: Around the top, between the back panels and the ceiling.You may wish to seal along the front, where the frame joins the tub/base and also the ceiling, but this is optional.