How to make a page into a collection page, without re-doing the work?

Hey, guys,

I’m making my agency website, and after I create the first project page, I realizes that I could use CMS for the projects.
So how can I use the page that I already create to became the project collection page template?

Thank you in advance!

Create a DIV under the body, drag all the page content into that div, copy the div, paste into collection page template?