Lower the blinds fully, using the pull strings. Open the blinds with the tilt wand so the individual slats are separated from each other.
Place a piece of masking tape on the slat that is even with the window sill. This slat will be the new bottom slat.
Turn the bottom bar of the blinds upside down so you can access the string plugs. Depending on the width of your blinds, you might have up to six plugs. Pry the plugs away from the bottom slat, using a flat-head screwdriver. Cut the string close to the plug with a pair of scissors, remove the excess string from the plugs and set the plugs in a safe place for installation later.
Slide the bottom bar out of the strings and away from the blinds. Set the bottom bar aside.
Measure from the slat with the masking tape down 6 inches, using a tape measure. Cut all of the strings at the 6-inch mark across the width of the blinds. Starting from where you cut the strings, the slats will release from the blinds, leaving you with a few intact slats under your new bottom slat.
Pull the center lift cords out of the slats below the new bottom slat. The lift cords are the center strings that go through the holes in each slat. The cords that run along the front and back of the slats with a small piece under each slat are ladder cords.
Slide each of the slats below the new bottom slat out of their respective ladder cords and away from the blind. This leaves you with the new bottom slat and 6 inches of string below the slat.
Install the bottom bar into the first set of ladder strings below the new bottom slat. Insert the lift cords through the holes in the bottom bar. Insert the ends of the lift cord under the bottom bar into the plastic plugs. Slide the plugs up to the bottom bar, but do not insert them into the bottom bar. Tie knots in the ends of the lift cords below the plastic plugs.
Pull down on the excess ladder cords below the bottom bar. Cut the cords below the second set of ladders beneath the bottom bar. Cut the connector cords on the two ladder sets below the bottom rail. The connector cords are the cords the slats rest on in the ladder cord.
Tie knots in the ends of each ladder cord. Twist the ladder cords together and insert them through their respective holes in the bottom bar from the top through the bottom. Position the ladder cords in the notches on the plastic plug, and push the plugs into the holes in the bottom bar. This secures all the strings. The blinds are now shortened.