WOW! I can’t believe I just spent two days on the simplest error with getting my Github pages to use a custom domain name.
First things first, here is the official documentation from Namecheap.
I followed all of the instructions including adding the records like so.
But still I was getting a 404.
Then I found this Stack Overflow topic and it hit me. I had the file name cname lowercased in my gh-pages repo.
Simply changing it to uppercase fixed my problem.
So make sure the file that has your cname is uppercased.