And then in your Zazzle account area click on the "Associates" tab. Or you can just click on this link http://www.zazzle.com/my/associate/associate to go to the same place.
In the picture below (from the Associates page) you will see what is my Zazzle associate ID to be used in this example. It actually explains there how to create your referral ID. Basically take the url address you copied earlier then write ?rf= and then your affiliate ID after (you can also use the "&" sign instead of a "?"). Or I, being lazy, will just copy and paste the blue part shown in the example that Zazzle gives and paste that after the url. so it looks like:
One of the advantages to creating a referral link manually like this is you can then refer other Zazzle pages than just a product page. You could create a referral link for the Zazzle home page.
Or a link to a search for certain keywords. Just do a search on Zazzle. Copy and paste what is in the url address bar, then put in the ?rf= and your ID. So it looks something like this.