You'll need to start by creating an account on http://bit.ly then you're all set to start shortening your long affiliate links.
To shorten a link while signed into your Bitly account, click on the "Create Bitlink" button for free users or the "Create" button for Enterprise users, in the upper right-hand corner of your screen.
From there you can paste in your long URL into the shorten box and click "Create". As a note, you can simply press 'b' on your keyboard to open this shorten box.
Learn about other ways to shorten a link, like our Chrome Extension and Bitmarklet here.
