Cover your work surface with a soft, clean piece of felt. This will ensure that nothing from your work surface scratches the acrylic.
Lay the acrylic on the felt-covered surface. Position it so that the area you are cutting is in or near the center of the surface top.
Mark the line you want to cut at both edges of the sheet. You can use a small piece of tape on the vertical edge of the sheet.
Line up a ruler or a straight-edged guide with both points to make the line against which you will cut.
Run the scriber down the side of the straight guide to score the sheet.
Score the sheet six or seven more times. You want to achieve a deep, straight cut. (You can slice all the way through the acrylic, but it is time-consuming.) Once you have sufficiently scored the material, you can apply another, more efficient kind of force to finish the cutting process.
Break the sheet at the scored line. The break should be relatively smooth, and the new edge of the material should be mostly smooth.
Gently rub any roughness on the edge with an emery board.