First, you'll need to download the Mailvelope extension for your browser — click on the link and then select your browser, then just install it.
Two, you know the bar where the extension icons usually appear? The Mailvelope icon will appear there, click on it and then on "Let's start!".
You will now see this screen:
If you don't have a key, click on “Generate Key”, if you have one, click on “Import Key”. Assuming this is the first time you've had contact with the subject, let's generate one!
When you click on generate, you'll be shown some fields to fill in: your name (it can be your nickname or anything, lol), the e-mail address you'll be using this key with (yours in this case), and the password to encrypt the key, preventing anyone else from using it.
A short break to talk about passwords, if possible, use a secure password manager (spreadsheets are not password managers). I can suggest options like Bitwarden, Proton Pass which are good online options, or KeePassXC which is offline, or Spectre which is an intermediary between online and offline, but doesn't have a database.
Sorry for playing the role of moral guardian of passwords, getting back to the point… Scroll down, and you'll see a checkbox saying “Upload public key to Mailvelope Key Server”, uncheck it (or don't, I at least recommend you uncheck it). Now click generate.
Once your key has been generated, you'll be in the “Key Management” section. This is where you'll find your private keys, as well as the public keys of your contacts (when you use GPG with someone, you download their key, usually a .txt or .asc file), and import it into this location.
Now, a small adjustment. Go to “Options” and then “Security” in the menu that will appear.
And under “Where are decrypted messages displayed?”, select “In a separate Mailvelope popup.” Just to avoid possible bugs (my own experience). And don't forget to click save.
Now, go to your webmail, after logging in (or logging in if you're already logged in), while in your email, click on the Mailvelope icon again, and select the option “+ Authorize this domain” and then ok.
Shall we test if it's working? Try writing an e-mail. If this pop-up appears, it's all right.
Click on it, and you'll see two fields: “Recipient”, where you'll enter your contact's e-mail address, and “Message”. Remember, in order to encrypt the message, the public key of your contact's e-mail must be saved in Mailvelope. To do this, click on the Mailvelope icon for the third time and go to "Keyring”, then just click on “Import” and load their key.
Well, that was the tutorial, the rest is intuitive and easy to use for most people, stay well, and use GPG/PGP. Honestly, I don't know what to use, because the right one would be OpenPGP, but they're better known as GPG and PGP. You know what? All that = potato.
Well, that's it, if you want my public key, click on the button: