Before anything else, go to the website poly.cam in a web browser. Not poly.com, poly.cam. In the top right corner, there is a black button with the text "Get started free".

Now you should see something similar to the screen below, with three buttons, each corresponding to different signup methods. Pick whichever one works for you. Keep in mind if you choose Google or Apple you are going to need to have the password for said existing account available on hand or memorized. if you choose to sign up via email, you will need to pick a password.

After you sign up, you'll need to answer a few questions for Polycam. if you don't feel comfortable putting in personal info like your name, as long as it says optional you can get away with not filling it out. Just click on the option that is correct for your situation; in the case below, if you are going to use it for fun or just for yourself, it would classify as "Personal". Once you're done, press "Continue" closer to the bottom of the screen.

For this next question, you can choose multiple options, not just one. pick only the ones that are correct. Once you're done, Press "Finish set up" at the bottom of the screen.

Now you will encounter a popup to sign up for a paid plan.  there's a small X in the top right corner of the popup; press it to close the popup.

You should now find yourself on the main screen.