Light Impressions sells a tearing rule for this purpose. However, it can be
done as follows: Rule oflightly in pencil on the BACK side the rectangle you
wish. Place a steel ruler on each line (with the paper flat on a table) and
pull up ... tearing the paper, using the steel ruler as your guide. Try to tear
each side with one steady pull. Do this on each side and turn the paper over
and, Voila! if you have done it properly, you now have four deckle edges.
Gene