During the process of buying you also create your account. After you enter your credentials and credit card information you will be asked to confirm your email. After that you will be able to login to the system.
The default page is your profile page. You may ignore it and navigate directly to Boilerplate to the left:
To connect your GitHub account to the private repository find your name entering it in the text box. After you find it, click the Connect button. The remote call will be done to the Github, and the access will be granted to you:
You will able to download the code source directly, or via the Github workflow, see the article here.