In the top right corner of your Yahoo Mail account, you'll see a Settings icon:
When you hover over that icon a menu appears and you want to click Settings. This brings up a popup modal with various settings options:
Click Accounts and then the email address where you want to set up your forwarding.
Down at the bottom of the modal you'll see the option to enable your forwarding and a field where you can paste your "...@inbox.groovehq.com" forwarding address.
Here you can also select whether you want to Store and Forward all messages or whether you want to Store and Forward and Archive all messages. The latter will help keep your inbox clean and uncluttered.
When you Save your settings, a confirmation email will be sent to your Groove mailbox:
Once confirmed you'll be good to go!