I would like to populate the database with existing passwords

Certainly. That's the main task of a password manager: create, use and maintain a database of credentials.
You have 2 choices to import existing passwords from wherever you currently store your passwords. One is the integrated import, see https://support.1password.com/import/
An advanced, more customized import, supporting a huge number of password manager products to import from, is community-developed by MrC: https://1password.community/discussion/101693/moving-to-1password-from-another-password-manager

I thought you were actually joking with "post-it", but if you do need to copy post-it's into 1Password, that's of course possible. Install the desktop app, press the big "+ new object" button and enter everything manually.

However, it's more easy if you add website credentials to install both the desktop app and the browser extension, then just navigate to the website, start to log in with your written down credentials and along the way save the credentials you just entered with the 1Password browser extension. This actually touches a small caveat of the browser extension: you need to explicitly save the credentials you just entered just before you submit the login form to actually login. Right-Click the browser extension icon->Save Login. This will create or update the login item for this website, if you are currently at the login dialog and entered your credentials.

This way you don't need to enter the website url, and 1Password saves everything needed to enable you to properly autofill the login details to that website later with just one mouse click.

I suggest you create a trial account and try the apps yourself. The trial is actually the full version, with full support, with 2 weeks gifted subscription. If you want to continue using 1Password, you just need to renew that trial subscription with a paid subscription and you are able to continue with the account you started.