Thanks for the comments. I'm looking for a DIY guide - something easy to understand, ideally with some photos, as I'd like to see what the various ventilation holes/channels look like. Also, recommendations for sealing products. If you can post including photos, that would be fantastic
There are CAD diagrams of suitable sections on the web somewhere. Sorry, I can't remember where but I probably saw the links on the greenbuilding forum!

swallow your pride and go look ... especially given how much you're planning to invest.
I'm planning a very large window - which is going to cost £1500, together with a heavy, large area of glass, which I'd like to be triple-glazed. I'm also planning to replace some old 6mm double-glazed units - some of which have blown - with triple-glazed units (and maybe new frames) - I want to make sure I get it right!
I intend to build a Passivhaus and I believe I will need triple glazing as a result but be aware that there are lively discussions about whether triple glazing is necessary or desirable in the UK climate even in a house insulated to the passivhaus standard. You haven't told us anything about your windows - orientation, shading, exposure, interior temperature etc - so there's no way to tell whether triple glazing is a good idea. Nor have you suggested the Ug, Uw or g values you want to achieve.
In addition to the above factors, you haven't told us what wood you intend to make the frames from, so there's no way to suggest what if any treatments are appropriate! Not that I have a problem with any of the names I've seen, but I'm no expert.