Well my manual that came with the car tells you where to place them on the front plastic and size etc. The manuals in the car... and a nasty lorry driver nicked them off, off my car when we got in on the ferry to drive off... but we got them back... only bits of black tape.
What I did do was to buy some headlight protectors [Altea spares] and to my surprise it came with deflectors as well and these stick inside on marked areas. You can re-use them and it does protect the lights & nastly lorry drivers from east europe can't nick them either.
I don't have fancy halogen bulbs [? err ay whatever] which I think don't respond too well to deflectors. Let someone else fill the gaps in on that one. Other people just adjust their lights down, illegal. Think the protectors with the deflectors thrown in is the best solution myself, accepting the bulb
problem that some people may have.