Add-on shower light fixture, have already run wire to existing single switch box.
What is the wiring to make double switch from the existing box? The existing switch has one house feed in/out

Here is a post with a diagram for three switches, see if it helps assuming only two switches.

https://www.askmehelpdesk.com/electrical-lighting/wiring-3-switches-each-controling-seperate-light-61713.html