Firstly, you should decide whether you'd like your blind to fit within the window recess or outside of it.
If you want your blind to fit outside the window recess, take three width measurements across the top, middle and bottom of the window. Next, complete the three measurements vertically, on the left, middle and right. For both sets of measurements, make sure to add around 15cm to ensure no light can leak through from the side or bottom of the blind.
If you'd like your blind to fit within the window recess, take the same measurements but from within the window recess. In this case, make sure to remove about 1cm from the width and length so your blind fits snugly in the recess.