To set up your domain, first log into your Namecheap dashboard and go to your Domains List.
Click Advanced DNS
Scroll down to Host Records
If there are any records in the list with the same subdomain you want (www), delete them.
Add New Record
Use the information on your Podpage Domain Page. Type is CNAME Record, Host will probably be www (unless you want a different subdomain), and value is the long value that ends with herokudns.com.
Follow the rest of the Podpage DNS Settings
Follow the instructions from this article to finish setup: