The best way to get your curtains just right is to install your tracks or poles before you measure up for your curtains. Start off by looking at your window to decide where you want your curtains to go. If the window sits in a recess, decide if you want the curtains hanging inside or outside of it. If you're putting your curtains in front or, if your window sits flush to the wall, place your poles and tracks 15cm above the window and extend by 15-20cm either side - this will make space for your curtains to be drawn back. If you're planning to have very full curtains, leave a few centimetres more to allow maximum light into your room.
If your window is set into a recess, it needs to be 7.5cm deep in order to fit the headrail.
Double-check how much space any window handles and hooks need and measure for the bind in front of them.
Measure both the width and the length of space to get the right dimensions. Remember to take measurements at different points and use centimetres if possible to ensure accuracy. If you prefer to hang your blind in front of the recess, position it as far above as you choose, then measure the inside of the recess crosswise and add a minimum of up to 4.5cm to each side.