When measuring your windows for new blinds the most important element to consider is accuracy. You will need to measure each window separately to the nearest millimetre using something that doesn't flex, such as either a steel tape measure or wooden ruler.
You will also need to determine whether your blinds are face or recess. Face is when the blinds hang outside of the window frame while recess is when the blinds sit tidily inside the window frame.
When measuring for recess blinds, accuracy is extremely important. From the inside of the frame, measure straight from top to bottom to find the height of the window. Repeat this in at least three different places along the frame and record the smallest of the measurements. Very few windows are perfectly square and over time can shift quite significantly due to age or moisture. Next, measure the width, once again measuring in three different places along the frame. Remember to record the smallest measurement to the nearest millimetre.
When your blind is being made the manufacturers will take a little more off this measurement again to ensure the blind fits snugly within your window frame. What they take off will depend on the style of blind and how it is fitted.
When measuring for face blinds you should still be accurate but it isn't as crucial. Generally you will measure from where the blind is to be mounted down to where it will hang for the height and measure lengthways across the window for the width. It is suggested that for full coverage and maximum privacy you allow for a 50mm overhang on each side of your windows.
For both recess and face blinds it is suggested that you draw up a diagram and mark in your measurements against it. It is also important to check all your measurements and ensure that everything is being measured in millimetres.