In article , news@pen-y-
geulan.com says...
snip
Answer for hole 1 - say a window sized hole
I do this in stages, I build the wall neer the top, say 1 brick away,
and let be for a day. THis stops it all moving as you mess arround in
the next step.
Next put in a final row of blocks, with a ridge of mortar at the back,
the ridge does not have to be neet, let it be a day
finally push mortar tight into the last bit of gap, the bits of ridge
of mortar you left will stop the stuff pushing straight through.

Answer for hole 2 = use final 2 steps for hole 1.

Finally I reckon a pro will do this differently.

I don't know about pros, but I've thought of maybe using a bit of UPVC
plaster bead or similar wedged around the edge of the hole to stop the
mortar from falling through the back.