I've never found Amazon to be particularly easy to do anything but order from. However, I think that this should work:
1) Search/Navigate to the product details page for the item you want to review.
2) At the top of that page, you should see a "customer reviews" link -- click it
3) Just below the "Customer Reviews" section header, you should see a "Create an online review" link -- click it
4) After entering your product review, scroll down to the bottom of the page and you should see a yellow "Preview your review" button. This is where I stopped, but I assume that if you've entered all the information they want you to and click the preview button, you'll get a preview page, probably containing a legal disclaimer, and a "Submit your review"-type button to click.