Here you will find some general guidelines to successfully measure your windows.
1. Measure with a steel tape measure, rather then cloth tapes which can stretch.
2. Do not use rulers or yardsticks as they are not very precise at measuring windows.
3. Measure all windows - even if you think some are similar - windows are rarely identical.
4. Take measurements for both inside and outside mounted treatments.
5. If you have an inside treatment, measure the recess to the first projecting item (ie window crank or handle)