These instructions are for adding a trigger button for a SINGLE engagement. If you wish to add fast checkout buttons for your entire online store, then please see Implementing the Instant Buy Button Code
The RXP provides you with two two code snippets to add to your website.
The first piece of code for the <head> section of your website. Depending on your website, you will only need to add this in once. As a general rule, add this code just before the </head> tag in your site templates.
The second piece of code is to copy and paste in where you want the button to appear.