To fix your blind you will need to take into account the direction the window opens, and to check that window catches will not interfere with its operation. With metal window frames ensure there is suitable surface onto which you can screw the fixtures.
Recess fixing:
To fit the blind inside the window a minimum recess depth of 7.5cm is required. Check the window recess is square by measuring the width top and bottom; if there is a difference, take the smaller of the two measurements. If the difference is more than 2cm a recess fixing is not recommended for a roman blind.
Face fixing roman:
Measure the window itself, and add a minimum of 7.5cm to the length and 12.5cm to the width, to cover the window adequately.