1. Click "edit HTML" button.
2. Make an IFRAME. Or better yet, go to Amazon.com and paste the HTML they give you as affiliate (this is what I do). It contains an IFRAME.
3. You can paste several IFRAMEs.
4. Put some text between the IFRAMEs.
5. Click "Update"
6. Save the post
When you view it, well... Your post is messed up.