If you want to place the curtain inside of the opening:
Measure vertically from the underside of the lintel to the bottom (floor).
If you want to place it in front of the opening:
Measure from 3cm above of the opening vertically down to the bottom (floor).
Take Measurements horizontally depending on how you want to place the curtain in the same way as above for the height.
For a Curtain with an Arch (a):
As shown in the diagram place a straight edge horizontally at the bottom of the arch and measure a.
Measure to the same point as you have measured for the height!
If in doubt, seek the help of an experienced person for taking/checking the measurements!