1) Login to your WP admin dashboard and select "Appearance" > "Widgets".
2) Click and drag the "Text" widget to the right hand sidebar.
the sidebar, click on the drop-down box and insert the HTML code that contains
your affiliate link.
<a href="http://youraffiliatelink">Enter Your Anchor Text</a> Note: Replace “youraffiliatelink” above with
your own unique affiliate link ID. Replace “Enter Your Anchor Text” with your
own text link. See below for illustration.
"Save" button and follow by the "Close" button once you’re done. Visit your blog now to see
the new changes at your sidebar.