Adding an associated wallet simply requires connecting another wallet and signing a message to prove ownership!
If you haven't read about what associated wallets are and how they can be used, you can head over that article here.
Navigating to add a wallet
Click on the identicon in the top right of the page
Click on your username to navigate to the Account Details page
Click on the "Wallets" tab
Click the "Add Wallet" button
Adding a wallet
Once you are the modal shown above, you simple just have to click "Add Wallet" and follow the steps outlined below:
Connect the new wallet you want to associate with your profile
Sign a Sign-In-With-Ethereum message to prove ownership of the wallet
Wait for the site to let you know that it was successfully added to your profile!