You link to another Hub exactly the same way you link to any other website. Open a new window, open the Hub you want, and copy the URL.
Then type the name of the Hub into your text, highlight it, click on the "chain" icon, paste in the URL and save.
You can use the "suggest links" feature, but I don't recommend it, because often the suggestions aren't that good. You should ONLY link to another Hubber's Hub if it really adds value to your own Hub. There's no benefit in linking to other Hubs just for the sake of it - the "Related Hubs" feature already does that for you.