That's a really good question PRT and one I've wrestled with for years.
I've tried a couple of times to lip it down over a resin / stone tray and then cover it with the tiles down the wall and to be honest I've no real confidence in the 'seal' you get as the lip is too narrow. Now I prefer to run the tanking menbrane [sheet type only] down the walls and below the top of the tray height. Then offer the tray up to it and secure the tray. Then - the most important, I seal the joint between the tray and membrane with quality sanitary Silicon and finish the top surface carefull so no pin holes or gaps. Allow to cure - 24 hrs and then tile on down to the tray, leaving a gap under the tiles of about 2 mm if possible. Then after all grouting [Do not get grout in the 2mm gap - clean out if this happens] apply second Silicon seal around the tray top, making sure the Silicon is under the tile edge. Baths are the same.
Others may disagree - this method works well and I can walk away with confidence. Job a good un. :8: