When you are editing / inputting an article in the Text box of the hub, you will find the editing assistance bar near the top (where you can select bold/italic/underline etc etc) and in that bar you will find a symbol like a chain link (a whole link icon and next to it a broken link icon).
To hyperlink a word or phrase in your article, select that word/ phrase, and click the "chain-link" icon. A box will appear there. If you are giving link to your other Hub, suggestive titles will be automatically listed there. You can pick one.
Else, you have to open (in a separate tab) the specific web page that you want to link to, copy its URL and then paste it here.
Once done, there is provision available in the same box for you to test whether your link is working alright.
Click Done. The job is over.