When editing a page, you can select whatever text or image you would like to turn into a link. Then click the button that looks like a globe with a chain. It'll pop up a window that lets you turn that selection into a link. There's options on the left, one of which is Page on This Site. That will show you all the pages on your site, as well as a folder that says Order Links. Click on that folder, then select the product you want to link to.

You use that same procedure for adding links to your privacy, contact, or any other page you have created.