After you log into your developer account, you should see a screen that looks something like the screenshot to the right. It's worth clicking around the interface to get a sense of what options are available. We're only going to focus on a small subset of the functionality.
Click on the "Create a preconfigured buyer or seller account". This will take you to a screen that looks like the next screenshot. On this page create a "Buyer" account. For the email, just enter your own - it will be converted automatically to a unique email address after you click "Create Account". For the password, enter something simple to remember - I chose 12345678.
At the bottom of the page there's a button highlighted in orange: "Enter Sandbox Test Site". This is the second confusing thing about this page. It seems like you're already in the sandbox site, but you're not! So if you were to try to put through a test submission through your form at this point, even though you'd see the URL in your browser point to https://www.sanbox.paypal.com, you're not actually in the sandbox. Weird? I know. Bear with me.