To measure the inside window recess you must measure from left to right, from plaster to plaster in 3 places; top, middle and bottom, choosing the shortest width.
You must them measure the drop from the plaster to the bottom of your window sill or floor area in three places, left middle and right side, again choosing the shortest drop.
If measuring the blind to be fitted on the outside of your window recess, then measure from left to right and top to bottom, but you must allow enough extra for overlap. We would recommend 6cm all round; assuming nothing is in the way that would stop the blind being fitted.
On Roller blinds, the exact width will be from the left bracket to the right bracket. In between the brackets you would have your controls, this means your material will be 2.2cm to 3.4cm less, depending on the width of your Roller Blind.