Cutting down horizontal window blinds may be necessary if you have measured incorrectly or if you have an odd-sized window. Blinds with wood slats can be cut to size fairly easily, but faux wood and standard aluminium Venetian blinds may be more challenging. Trim the headrail to fit into the window opening, as well as cutting down the slats and lower valance.
Remove the metal stiffeners from both ends of the headrail. Wrap a layer of masking tape around the top and sides of the rail.
Measure the window opening with a steel measuring tape. Halve the difference between the width of the new headrail and the width of the window frame and add 1/16 inch for clearance. Transfer this measurement to each end of the headrail and mark these points on the masking tape with a pencil. Double-check by measuring the distance between the two marks. Use a carpenter's square and pencil to scribe a line across the top of the headrail and down each side.
Cut the headrail ends with tin snips if the metal is not too thick. Alternatively, wrap a cloth around the headrail and clamp it in a vice. Cut through the ends of the rail carefully with a hacksaw fitted with a fine-tooth blade. File the ends smooth with a metal file. Replace the end stiffeners and test-fit the headrail into the window opening.
Align the ends of the slats and tape them firmly together with masking tape. Clamp the slats and the valance together with adjustable clamps. Position the clamps about 6 inches from the end of the slats with the clamp handles facing the headrail.
Mark the cut line on the masking tape. Allow 1/4 inch clearance in each side of the slats and valance.
Install a fine-tooth carbide-tipped blade into a table saw or chop saw. Position the slats and valance across the mitre gauge and move the headrail out of the way of the blade. Position the inside edge of the blade against the outside edge of the cut line. Turn the saw on and cut slowly and carefully through the taped bundle of slats and the valance. Take care not to touch the blade against the headrail. Turn the blind over and complete the cut on the other end the same way.
Smooth the ends of the slats and valance with a metal file. Remove the clamps and the masking tape. Use the file to round off the corners of each individual slat. Hold the file at 45 degrees and run it across the end edges of the valance once or twice to remove any burrs.
Wear eye protection when cutting horizontal blinds