Measuring for the length:

Measure from the top of the curtain pole to the place where you wish to see the bottom of your curtain.

Measuring for the width:

The basic approach recommends to multiply the width of the window by 1.5  allowing a little gathering, so the curtain looks more full.

For Thermal curtains to be effective, they must be installed properly.

Thermal curtains should be installed as close to the window frame as possible. They are most effective when they drop to the windowsill or floor.

Installing a cornice or valance above the drape further reduces the infiltration of air.The basic approach recommends to multiply the width of the window by 1.5  allowing a little gathering, so the curtain looks more full.