Step-to-step instruction for an absolute beginner

Hello there, All I want is to install Dropbox via the Flathub on my new Chromebook. Reason: I can’t read my LibreOffice files stored on Dropbox if I use the Dropbox version accessible via Google Play.
I managed to install Flatpack and LibreOffice via the Linux terminal on my Chromebook. Since I had never used Linux before, it took me several hours to piece together mostly dated or incorrect information on the web.
The same applies to advice and instructions regarding Dropbox. The Linux terminal is up-to-date, I downloaded the Dropbox installation file from the Flathub page, ad I typed the line below the Dropbox image into the terminal. Doesn’t work.
PLEASE, I need an exact and correct line that I can use in the terminal! After that, I hope I need not use Linux again!
Thank you for your time and smile
Paeone