This instructable comes in a UK based Metric Flavour.
In the UK, you also need building control approval for the installation of windows - you only need someone to come out and review the windows when you're done.
These are my first two, I'm not an expert and if I've done something you'd consider incorrect, let me know in the comments, but I'm pleased with the outcome and I'm now draft free.
This ible doesn't take you to the conclusion of fitting trim and doing the silicone sealant to make it all look pretty because we're not done decorating at the time of writing. I'll give you an update when it is!
The windows I'm replacing are Crittall Windows (http://en.wikipedia.org/wiki/Crittall_Windows_Ltd
) named after the company, and are used in North America and other places in the UK.
They're also the biggest pain to replace, and there's not a lot of information on doing it that isn't in video form. enjoy.