With something like this, I find it best to start with an existing template and then modify it to fit. I advise checking out Evil Ted, Punished Props, or the litany of shops on Etsy that sell these types of chest templates. Find a template that is close, print it out, and then make a paper mock-up of it. If any of the shapes need to be changed, you can draw directly on the paper and then cut it apart again on the lines that you drew to use those pieces as your new template.
Past that, follow assembly instructions from videos from the 2 makers listed above. They are some of the most knowledgeable makers available when it comes to foam construction.