Genuine question - what's the 'right' way of fitting them?
No easy answer to that one mate as a lot depends on who measures them up EG if they are measured up wrong & the gaps are too big then you end up with plastic trim around the edges & that to me is wrong, a 3 to 5mm bead of frame sealant is all that should be needed no more than that, expanding foam is not needed if its done right. measuring under to make fitting easier & quicker is common practice hence all the crap trim you find on 99% of houses, doing it right leaves a much better finish but takes more time
Sills should have a bead of silicone before being screwed to the bottom of the frame but rarely are (note that is similar but not the same as frame sealant)
A minimum of 3 frame fixings a side are needed for a door & one with a window on the side will usually need one or more through the head as well, proper plastic wedges should be used rather than bits of wood or other crap & the fixings should be tightened to them to insure the unit stays square, just hammering in a frame fixing isnt good enough. Getting the frame square is the hardest part of the whole job, particularly with doors
Inside reveals should be plastered to take up any gaps, bits of plastic trim are not acceptable in my world, I have spent hours removing them from my previous houses & doing the job properly, plastic window boards are ok if fitted correctly, plastered in & sealed to prevent any drafts from the cavity (if there is one)
Finally glazing units should be fitted on spacers to allow for expansion & contraction, too may times I have seen a double glazed unit sitting way down in a frame or over to one side because someone couldn't be arsed to space it properly (if at all) these are usually the units that have the seal broken & end up with condensation between the panes