Students first learn about using vernier calipers in Sec 1. It’s not easy to understand how it’s read immediately, although learning how to read it is mechanical and quite obvious. So I made a template which can be printed and cut out of cardboard, so you can make you own.
Print out the template. Cut it out.
The longer piece can be made out of thicker, stronger cardboard. The shorter piece needs to be folded and also to reduce parallax error, should be made out of thinner cardboard. Cut a hole out for the window.
Place the shorter piece behind the longer piece and fold it over. Stick it in place with glue or tape. Stick the shorter sliding rule to the bottom of the rule.
Assemble the two pieces and slide the jaws closed. Align the longer rule and then stick it down with glue.