In article , "Paul S"
says...
I will shortly be changing the position of my electrical sockets in one of
my bedrooms. Currently they are surface mounted sockets, but for neatness I
would like to take this opportunity to change them to flush fitting sockets.
When I have previously attempted to fix back boxes into walls on previous
jobs, I have always had problems getting the box square, and sitting flush
in the wall. I have previously only used a drill, hammer and bolster to
create the hole for the box. Recently I have seen these Box Sinker Sets that
fit to an SDS drill to create a perfect hole and they also are used to
channel the wall out for the cable, has anyone any experience of using
these? If so, are they worth the investment? (not just for one job), or has
anyone got any other tips for me to make the job easier and neater.

Make the hole too big (and too deep), whack in some Polyfilla, press the
box into position and support it with something until the filler sets.
The last bit is the trickiest, so figure out how you're going to do it
before you mix the filler :-)