Pretty straight forward here but here’s a quick walk-thru.

When you ‘Reply’ or start a ‘New Thread’ you can choose to subscribe to that thread by clicking on the ‘subscribe’ button. You can see it here:

You can manage you subscriptions by going to the User Menu/Forum Profile. Once in your Forum Profile there’s a menu on the left. You can access your Subscriptions and favorites there.

Also, so long as your signed in you can directly subscribe or favorite any thread at the very bottom of the thread