Summarize:

You need this to determine the exact length that you need for the mini-blinds. Start with the inside top of the casing where the blinds will be mounted, and measure all the way to the windowsill below. Measure from the top of the blind casing to the blind slat that most closely corresponds with the window measurements. Move downward 1 additional slat to allow a little room for error. Mark the first slat to be removed with a marker or pen. Mini-blinds are equipped with plastic plugs on the underside of the bottom rail. Most designs will feature 3 plugs along the length of the rail. Releasing the plugs allows access to the lift cord and the 3 string ladders on the mini-blinds. You can usually just pull out the plugs without having to do anything fancy. This action will slide the lift cords through the holes in the bottom rail and the slats immediately above the rail. Continue to pull the cords upward until the marked slat is freed from the cords. This will provide you with better access to the blind slats. With the lift cords no longer threaded through the slats, this requires sliding each of the slats out of the 3 string ladders.
Measure the height of the window. Place the mini-blinds on a flat surface and expand to the full length. Remove the rail plugs. Untie the knot on the lift cords and pull upward. Slide the bottom rail out of the string ladders. Remove the desired number of slats.