Most blinds are installed to order and fit the height of the window, but others aren't exact and scrape and rub the window's sill. Both vertical and horizontal blinds are adjustable after installation, either to fit perfectly or for reinstallation into a shorter window. The process cannot be reversed, though, as it involves cutting the string ladder of horizontal blinds and the vanes of vertical blinds.
Remove the plastic plugs from the underside of the bottom rail. Pull the knotted lift cords through the plugs and cut the knots off using your scissors.
Slide the bottom rail out of the string ladder.
Measure the height of the window with a measuring tape and locate the lowest rang of the string ladder that will reach this length.
Remove all the slats from the string ladder below the point measured for the length of the window. Place the bottom rail in the first empty rang of the ladder and confirm that it hangs correctly when the blinds are fully lowered. If so, cut away the rungs below the bottom rail. If not, add or remove rungs until it does.
Thread the lift cords back through the bottom rail and plastic plugs. Tie new knots in the lift cords to secure them to the plugs, and snap the plugs back into the underside of the bottom rail.
Measure the length of the window with a measuring tape. Subtract 3 inches from this measurement to allow the blinds to open and close properly.
Mark both edges of each vane with a pencil at the desired length. This will give you two points to cut across to keep your cuts level.
Cut through each vane with scissors from one pencil mark to the other.