Use a metal measuring tape and double check all measurements.
Inside a recess
To measure up for blinds fitted inside a window recess, measure width (A) at several points (top, middle and bottom), to allow for variation and make a note of the narrowest measurement. Deduct 1.5cm from this measurement to give clearance for the blind to move up and down without snagging on the sides (round your measurement down to the nearest half centimetre).
Measure length (B) and round down to the nearest half centimetre. Do not deduct anything- this will be the finished length of your blind.
Outside a recess
First, decide how much you want your blinds to overlap the wall or window casement (C). This is a matter for personal taste, but I suggest it should be the same width as the windowsill.
For the length of the blind (D), I recommend the blind be set 10-15cm above the window, and extend to 5cm below the sill.