The first thing you have to do is create your button in PayPal. They have very good instructions on how to do so. Once you have added what you would like, they will give you a piece of code that you will need to insert into the specific page you are looking to add the payment button to.
Once you have the code, just go into the page you want to put the PayPal button on. I would recommend that you first type something like, PAYPAL BUTTON HERE, that is easy to see. Once you have done that click the button that says HTML in your editor, find the PAYPAL BUTTON HERE that you typed into the editor before pushing the html button. Highlight that text and paste in the code that PayPal have you. The PayPal button will now show up where the text you typed in was.