Measure across the width of the roller blind, along the top edge. Draw a line with the marker at the point on the roller that's equal to the width of your window. Also measure along the bottom edge of the blind and make a second mark at the point that equals the width of your window.
Unroll the blind completely. Draw a straight, vertical line connecting the marks on the top and bottom of the roller.
Cut the excess blind fabric off using the scissors. Trim until you reach the top, where the roller is. The fabric of the blinds will still be attached at this point.
Put on gloves and goggles for protection. Use a hacksaw to cut the metal roller away from the blinds. Cut the metal at the mark you drew, using straightforward motions. Once you cut all the way through the metal, discard the excess roller and fabric. Hang the blinds in your window using mounting hardware.