Subdomain of website for Github pages project

I have a Github project, github.com/jeti/matrix, and I set up a "Github pages" site for the project so that it is accessible here jeti.github.io/matrix/.

That is all configurable through Github.

Now, I would like to add a subdomain of my personal website so that the website is accessible via the subdomain matrix.jeti.io of my website jeti.io.

I am just really confused how to do that because the documentation don't seem to show how to redirect a project page to a subdomain. I have tried a few permutations of what I think should be the correct inputs, but because these DNS changes take so long to propagate, it is really hard to test.

Specifically, I would like to know what value to enter into Github as the Custom domain (it seems to me that this should simply be the subdomain matrix.jeti.io, but I am not sure, so I have left this blank):

Then I also need to create the subdomain. I bought the domain through OVH, and they provide a few options for adding a DNS entry:

My understanding is that I need to add 2 apex records. I did that already:

What is unclear to me is whether I also need to add a CNAME entry. This is what the form looks like when I try to add a CNAME entry:

So in recap:

1. I did not specify the Custom Domain on the Github site.
2. I created the 2 apex records shown above.
3. I did not create a CNAME entry.

Please tell me which of these steps needs to be changed and how to modify it.

After more trial and error, the answer seems to be

1. The Github custom domain should indeed be matrix.jeti.io

2. I did not need the apex records. In fact, Github emailed me discouraging it. So I deleted the apex records.

3. In OVH, I added a DNS CNAME entry like this:

The thing that was confusing me is that I thought that the CNAME entry needed to have a link to the original Github pages WITH the project name jeti.github.io/matrix. That was wrong. The target is simply jeti.github.io. (Note the period on the end).