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.
Step 1: Measurement & getting a quote
Take the smallest of the three measurements and deduct 10mm. This allows a 5mm wiggle room.
I went for very simple windows which open on one side.
Make sure you draw and label what you want, and make sure you identify which measurements go with each window. Don't assume all the windows are the same size. My single glazed were lined with wood which had been adapted to the gap.
Then, once you're done, go out and measure again. There is something called the 100mm madness where people add 100mm to the dimensions, especially if they're not used to working in metric.
Ring round a few companies and get the best price. What you need to look out for is whether the units are welded (most if not all are), that they're as efficient as you can afford (A rated). Ask for supply only.
Pop in and see what windows they offer - you might not like the style or fancy extras like wood effect. I went for bog standard cheap white PVC.