In message . com,
writes

I need to replace a wooden 'Georgian' style window that is rotten. I
know how to get it out but have never fitted a double glazed unit
before. How does the frame fasten to the wall to make it secure- the
old one simply has nails driven through and the heads filled and
painted over.
Is this how a wooden sealed unit fixes ?

you could nail it, but the normal way is to use frame fixings. basically
drill a hole through the frame, then one into the wall . Insert fixing
and hammer home - IME you will want to countersink the hole to take the
head of the ;ug and fixing if want a neat job. Seal around the frame
with expanding foam and finish off outside with bead of sealant.

Does the glass come out first
and the refix it in the frame after fitting that ?

Yes, in fact I'd expect it to be supplied separately - mine was.

The glazing can be fixed in various ways. best is to use glazing tape
(or whatever it was called, basically a self adhesive tape) and gazing
silicone. though you can use silicone or there is a mastic you spread on
with knife you can use which I found total pain to use.

Is it a DIY or have I to call in the 'Professionals" ?

I've DIY-ed two, though I wouldn't suggest it as a first job. It's not
particularly difficult job, though can be a bit awkward at times.
--
Chris French