When you're writing a hub, use your mouse to highlight a word that you want to use as a link, then click on the little icon that looks like a chain link. It will be above and slightly right in the hub capsule. A window will open. Copy and paste the url (web address) into the correct entry (it's the top line), then save. You can also type it in and then save. After you've published the hub it's a good idea to visit it and click the blue letters to see if you are taken to the site you wanted. You know, test it.