The paypal script comes with a function call that creates buttons for you. Example code to show a variety of ways to implement your paypal buttons. Also included
is a sample return url page, aka the thank you page. And a sample script to handle Instant Payment Notification (IPN)
After you have downloaded the zip file, you will need to upload its contents to your web host (must support PHP, most do). Using FTP or whatever you normally use to get files onto your host.
Upload the files to any directory, and view example.php in your browser. Edit the example.php and other files to customize and improve your checkout pages.