Assuming you have already bought a custom domain (example.net) and a Github Page set up (example.github.io):

Create a CNAME file

Create a file name CNAME in the project's root folder with the custom domain example.com inside.

# Outputs example.net to a file named CNAME
echo example.net > CNAME

You may need to configure your build command to also copy CNAME to the build folder, for example with a package.json file with React:

  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build && cp CNAME build/CNAME",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  }

Configure your domain

At your domain provider (the place you bought the domain - which I recommend porkbun.com), find the configuration for custom host/resource records and set:

Type: A
Host: @
Answer/Data/Value: 185.199.108.153
TTL: 30min (or any)

Do this for all 4 github page ip addresses: 185.199.108.153, 185.199.109.153, 185.199.110.153 and 185.199.111.153

You'll also need to add this one:

Type: CNAME
Host: www
Answer/Data/Value: example.github.io
TTL: 30min (or any)

Configure repository

At Github, go to the repository settings and add your custom domain in the Github Pages section.
It might take up to 24h to reflect these and/or HTTPS changes

References:

• Managing a custom domain for your GitHub Pages site