how do you use a ruler to weigh a bass, i was told once but i forget??

any weighs better than length times girth times girth divided by 800.

A ruler is an instrument used to measure items - you can't weigh with a ruler.

how do you use a ruler to weigh a bass, i was told once but i forget??

There are length-weight charts that can be used to estimate weight, but fish vary so these charts can be accurate or way off. New York uses the term "weighing fish with a ruler" in an attempt to get people to actually weigh fewer fish because of possible harm to them and to estimate weights instead. I personally weigh only the fish that are "big" by my standards and use length for the rest.

Here's the New York charts:

http://www.dec.ny.gov/outdoor/9222.html