Vertical  blinds for windows typically come in a standard length of 84 inches.  Some manufacturers and distributors will custom cut your new vertical  blinds, but often for an additional fee. Low ceiling height, the  installation of an exceptionally thick carpet, or the installation of a  new floor over top an existing floor are all reasons why a length  shorter than standard may be required for your 
vertical blinds. You may  wish to trim a set of existing blinds when relocating them from one  window to another. The basic technique for cutting 
vertical blinds  varies only slightly according to the material with which they are made.
Instructions
 Fabric Blinds (No Vinyl Backing)
Remove 
vertical blinds from rod and lay on bare floor or other suitably long, flat surface.
Remove weights from bottom of fabric louvers and set aside.
Measure  each louver lengthwise and mark with pencil. Allow enough extra length  to fold and re-sew the weight pocket (approximately 2 1/8 inches extra).
Cut each louver individually with scissors and straight edge where marked.
Fold each louver under, and sew to create weight pocket.
Vinyl, Perforated Vinyl, Metal or Vinyl-Backed Fabric Blinds
Put vertical blinds on a flat surface.
Measure each louver for desired length. Mark with pencil.
Cut each louver using scissors and straight edge.
Sand louvers with minimal pressure to eliminate rough edges. Start with a fine grit and end with superfine-grit sandpaper.
Faux Wood or Wood Blinds
 Lay 
vertical blinds on a flat surface.
Measure louvers and mark with pencil for cutting.
Cut each louver individually with a chop saw.
Sand  bottom edge of each louver, beginning with fine-grit sandpaper and  ending with superfine grit. Use only enough pressure to sand away rough  or uneven edges.