This is a former I'd just laid. The corners made up in sand and cement where an old pipe went through. The light grey stuff is the tanking primer which I cut in prior to fixing the tray into position. That just gave me a nice clean surface to bond the tray to.
This shot shows the stud wall in place, blame the bathroom showroom for not having the tray the full width, and with the first coat of tanking applied. After the first coat was tacky, I then applied the 4" strip ( as seen with Roman written on it ) into a layer of wet tanking. That then gets brushed quite firmly to remove excess tanking from behind the strip to ensure it's as flush and flat as possible so as not to hinder tiling. The strips run between the former and the insulation backer boards on the floor ( insulation boards as this had undertile heating on a ground floor ( concrete )), in all the internal angles both horizontal and vertical, and around the turn of the boxing in eg the external also.
Leftover strip used around the shower valve. Belt and braces, aka OCD.
Finished article prior to grouting. Note the shower valve is dead centre of the border tile ribbon, and also there are two same size tiles either side equally intersecting the shower valve. That's gives the same size cut of tile around the entire valve plate. Aka OCD
All of the above can also be observed when fitting a shower tray, just the only difference being that you can't use the strip between the shower tray and the wall. As the tile will only cover 12-14mm of the edge of the tray, you can't have the strip showing so another change of discipline is needed. The next one is a one piece shower tray, so basically you just tank the walls prior to the tray being fitted, which gives an excellent surface for the adhesive ( mastic type ) to adhere to when bonding the tray into its final position. What the picture doesn't show is the masking tape that I put onto the tray to allow me to get a neat edge to the sealant where it sits on the tray edge.
That junction is quite important as it's the last line of defence against any water getting behind the tiles. What I do I these circumstances is to pump a load of mastic ( sikaflex for these is best as it's non silicone based and also doubles up as a sealant as well as an adhesive ) between the tray and the wall and then push the tray against the walls thus displacing any excess above the tray edge. This is when you'll be glad you masked it up first . Get a plastic spatula and then spread the excess sealant, whilst still wet, against the vertical wall face. Do this so you get an up-stand of sealant forming a 'skirting board' which is continuous between the tray, the wall, and vertical face of the wall, preferably all in one application. I set the masking tape around 5-6mm in from the edge of the tray so I know I'll get a straight line, and also a bead of sealant that's has a decent purchase on the tray, which then transfers to the wall. The action of tooling the sealant with the spatula pushes it against the tray itself thus forming a good / strong bond to both. After the horizontal sealant has cured, next run a bead up the internal angle for at least 600-900mm up the corner.
With the fundamental sealing done, in goes the 45'd shower valve & pipework frame sections which them get plasterboarded and sealed, plus tanked prior to tiling.
Roberts your mothers brother
The end.