I think I've read all the subjects of this nature, but I'm after a few
more specific details.

I am installing a downstairs shower room. Entry wall (with doorway) and
adjacent right wall are plastered with various patches here and there,
which make them uneven. The rear wall and adjacent left wall are of
12mm WBP, which I installed over timber studs. The rear wall for
separating the room from the old galley kitchen that it was once part
of and the left stud wall was to create a space to take waste from the
toilet, past the shower tray and out the rear wall. Also to conceal the
toilet cistern, allowing for a back-to-wall pan.

The WBP walls are pretty flat, although not perfect... one stills seems
to have a 3-5mm dip, horizontally across it. The plastered walls are
pretty dreadful with belly-outs at the top and bottom, giving a big dip
along the height of the wall.

I intend to use a 6mm notched trowel, as the tiles I'm using are quite
big (400mm x 250mm) and I want to set them in a brickwork pattern. From
experience, I know it's hard to combine a combing application of
adhesive while trying to also use it as a filler for dips in walls. I
can see how it would make sense to find the most protruding part of the
wall and set a tile on it with a standard 6mm combed bed and then add
more adhesive to the areas where the wall dips away, thus keeping the
facsia of the tiles aligned.

However, what's the technique for applying both a background filler
with a combed top layer allowing accurate setting of the tiles and does
one do it tile by tile or over a larger area? I know that doing away
with a combed layer means trying to push excess adhesive out around the
edges of the tile which is almost impossible and very mucky.

Alternatively, as suggested elsewhere, would it be better use a
straight-edge across the high points of the walls to scrape away an
application of one-coat plaster, thus bringing the dips and shallows
out to the same level with a deposit left behind by the straight-edge?
A plasterer I am not, but then a glass-perfect finish is not what I'm
after here is it?

Any advice would be great, to perhaps fully cover this subject and not
have it repeated by someone else further down the line

Thanks much.

deano.