If you are drilling into tiles make sure you get a proper bit for the job. My advise DON'T use a masonary bit, get a special one for tiles/glass. I think there are about €7 for a 6mm, but well worth it rather than cracking a tile. Mark where you want to drill, cover it with a couple of strips of masking tape in an X fashion (in case bit slips) mark the masking tape, u'll be able to see the original mark but easier when holding the drill to have mark prominent on tape. Drill away! Best a luck with it.
Some stores will tell you a masonary bit will do fine but if u are a novice use the proper bit.
Just reread your question, you are going to need advise on sealing as well, not on as solid ground here but - I think ensure the lip of the enclosure is inside the tray so that any drops fall inside the shower and then silicone inside and out.