When on a project to fit windows, the first thing you need to do is measure the glass. You will of course need to decide on where you are going to get this glass – and consider talking to them for advice with regard to the kind of glass it is that you are looking for.
You may need to order glass from the manufacturer – but you should know this in advance.
Apart from that, establish the daylight glass service using a tape measure; the points to be accounted for are at the top, the centre, the bottom, the left, the middle and the right. It is standard procedure to state the width first when ordering glass for windows.