To get started with using HTML5 Validator to check your HTML you will need to create a .yml file at .github/workflows/ in your repo.
To learn more about getting started with Github Actions, check out the documentation.

An example of a simple configuration would be:

name: HTML Check
on:
  push:
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2 # Requried will all actions
    - name: Checks HTML5
      uses: Cyb3r-Jak3/html5validator-action@<latest version>
      with:
        root: <path/to/your/files>

to see all options, check out the inputs in the README.md.

You can also check out the 3 workflows that I use with this project here. The most useful one is going to be the action-test.yml.