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Typically, a set of inside-mount blinds should measure about 1⁄2 inch (1.3 cm) less than the width of the window frame. Make sure you measure the inside of the frame, where the blinds actually fit, and not the entire window frame. Measure in at least three places -- the top, bottom, and middle of the window. Most windows are going to have regular, even spacing, but older houses may have some variability. If there is a change, use the smallest measurement taken. This small gap will keep your blinds from getting cut or stuck on the windowsill. Use the measurement found above and mark the headrail (the top of the blinds) at this length. You may either cut an even amount off of both sides or cut only from the pull cord side. If the chords are on one side, you can cut only from the other side. In many ways, only cutting one side is much easier. Simply start sawing to remove the headrail where you noted:  Remove the metal end stiffener bracket from the side(s) you are cutting. Use the hacksaw or tin snips to cut the headrail per your measured mark. File any rough edges with a metal file. However, keep in mind that the headrail edges will be covered by the mounting brackets, so a rough cut will not be visible. Replace the metal end stiffener onto the newly cut headrail.

Summary:
Measure the width of the window to determine the necessary width of the blinds. Mark the smallest measurement, and cut to this. Subtract 1/4-inch from each side to ensure the blinds have room to move up and down. Measure the blinds from the head rail and use a pencil to mark your new desired length. Cut the blind headrail using a hacksaw or tin snips.