Go to My Account
Click the Earnings tab
Select Referral Trackers from the menu at left
Either make a token using words, or use the one provided that's all random
Plug your tracking token into the url below, making sure to put the "_" underscore before it
After you have that url made up, write the words on your profile that you'll want people to see: something like "Sign up here" or whatever. Select (highlight) that text when you're in edit mode, hit the link button (that looks like an infinity symbol), and insert the Tracker URL you made above. Hit Insert. Save. Voila!