Step 1: Log into Blogger and go to the Layout tab and click the "Edit HTML" sub-tab.
Step 2: Backup your template. Click "Download Full Template" and save it somewhere on your hard drive in case you ever royally mess up your template and need to start over fresh with a known-to-function template.
Step 3: Click the "Edit Template Widgets" check box. This lets you see all the template code you'll be adding your HTML code to.
Step 4: Search for "data:post.body" in the template using your browser. That bit of text signifies the end of the body of a post in your template. You'll be putting your HTML code directly after that.
Step 6: Click "Preview" and make sure it worked as expected. When you're satisfied with your footer and there are no HTML errors click "Save Template".
Piece of Cake!!
See also: How to Add an Amazon Widget to Your Blog to Make Some Extra Cash