After pressing Enable a new button appears for connection between your Favro account and your Microsoft account.
Clicking Connect with Microsoft opens a new tab where you log in to your desired Microsoft account.
After log in you will have to give Favro permission to access the your Microsoft information.
Back in Favro you'll see that your accounts are now connected.
Clicking the Files drop-down will now have a OneDrive option.
Clicking it will open your OneDrive in a new tab. Choose the file you wish to attach (you can attach multiple files at once if needed) and press Open in bottom right corner.
After pressing Open you will be taken back to your card inside Favro. By pressing the menu button on your attachment you can choose to delete or open the attachment in OneDrive.