How do I install Holder AND work on a second theme?

Hello! I basically want my visitors to hit a Holder page for a bit while I work on a theme that will be around for a while longer. How do I do that? I have Holder installed and I'm working on it but now I want to start working on the stuff in the background where nobody has to see my dust.

Also, I have read about parent and child themes, and I would like to know where the instructions are for setting this up. Naturally I don't want to overwrite any customizations I do to my theme if an update becomes available.