There are a couple ways to do this.
My way is not documented. It is to have EVERYTHING for each project in a designated and dedicated project folder. That includes copies of all video, image and audio assets as well as all the Scratch Disks (from Edit > Preferences > Scratch Disks). I can then copy the ENTIRE folder anywhere for storage. Then I can copy it back to the working drive for future edits. Note that this may not work if you upgrade to the next version. You have to keep your various version to match your projects.
Another way is to use the Project Archiver.. tool in the file menu. I've tried it a couple times and it works. I don't use it because I have more control over folder structure when I do it may way. As I recall, there are two choices in the Project Archiver process. One saves a larger set of files for future work. The other trims up files to save space but leaves fewer future editing options. Note that a few versions back there was a suggestion on this forum that there might be a bug somewhere in the Archiver. Before you delete your master project folder you might personally test being able to open an archived project.
Please refer to the user manual ( https://helpx.adobe.com/pdf/premiere-elements_reference.pdf ) for explanation of the Archiver process.
Thank you for your advice!