I have a giant teaching slide-ruler I got from my father in-law's estate. It's missing the slider, and needs a good cleaning. Oddly enough, if you look closely, 2 *3 <> 6, the scale is slightly off. ;-)

Slide rulers are pretty easy to use, and make.

This Illustrated Self-Guided Course On How To Use The Slide Rule [1] seems reasonable.

[1] https://sliderulemuseum.com/SR_Course.htm