Set custom domain for user and project pages - github-pages

Question

I would like do something like:
mydomain.tld (apex) redirect on www (user page)
www.mydomain.tld (www custom) on user page
myproject1.mydomain.tld (custom) on project page
Actually I've a DNS zone like:
# 10800 IN A 192.30.252.153
# 10800 IN A 192.30.252.154
www 1800 IN CNAME user.github.io.
But, I don't know how to add a custom domain for project pages, because I can't do a CNAME on user.github.io/myproject1...
In Github, I've set:
user.github.io repo: www.mydomain.tld
myproject1 repo: myproject1.mydomain.tld
Actually it works for www.mydomain.tld, I think it's okay for mydomain.tld too (just wait propagation). But I don't know how to do for projects pages.

Answer 1

In Github:
Define the domain you want to use:
In user.github.io repository, define the: www.mydomain.tld
In projects repository, define: project.mydomain.tld
In the DNS zone file:
# 1800 IN A 192.30.252.153
# 1800 IN A 192.30.252.154
project 1800 IN CNAME user.github.io.
www 1800 IN CNAME user.github.io.
In the configuration, we use CNAME to define subdomains, and the A on IPs: 192.30.252.153 and 192.30.252.154 to define the apex.
Then:
www.mydomain.tld serve the user.github.io pages
mydomain.tld redirect on www.mydomain.tld (alias)
project.mydomain.tld serve the user.github.io/project pages
Because apex is define, we also can do:
project 1800 IN CNAME #