You MUST fit the channel onto a wet bead of CT1 / clear silicone ( I only use CT1 for this as silicone has a limited life ) otherwise water collects inside the channel and will find its way between the wall tile / panel and the tray and soak into the wall. This has to be the most common F up I see on a lot of jobs, mostly insurance claims after it's gone rotten   Offer the channel up dry and hold it in place, level, and use bits of masking tape as runway markers, mark and drill holes, clean