Sealing shower tray

I am about to install a shower tray in the same posiition as the old one I am removing which has leaked slowly over last two years, just causing damp ceiling.

The tiles are in reasonable condition although a bit irregular where reach shower tray; how can I seal the tray when fitting new base, i.e before silicone sealant is applied as last job?

Silicone the edge of the tray, that is going to fit to the wall.
So you get a good seal alway around the edges.
So apply silicone then push the tray up tp wall and then secure tray, I'd personally think about removing the tiles to, and fitting new, as this will help to create a better seal. As you can grout and silicone it, around the bottom edge of tray.

Also I'd leave a 2-3mm gap from the bottom of the tile to the edge of the shower tray, gives enough room to fill with silicone.