Measure a distance of 12 inches above the radiator. Mark the point lightly with a pencil.
Measure the distance from the curtain rod down to the marked point above the baseboard.
Purchase or unearth curtains that are not longer than the measurement you took. If you cannot find curtains of this length, you may need to buy longer curtains and hem them with a machine or by hand.
Slip the curtains over the rod and mount the rod onto the wall above the window as you usually do. This may require slipping the ends of the rod onto wall brackets, settling a straight rod onto a pair of protruding brackets, or sliding an expandable rod to rest snugly at the top of the window well.