This assumes you have already installed the Home Assistant Community Store following the instructions
https://hacs.xyz/docs/setup/prerequisites
If you want other kinds if installation, please see README.
In your HACS store, look in the Integrations page. In the top right corner select the 3-dots menu, and choose Custom repositories. Paste in the URL to this repo,
https://github.com/ha-warmup/warmup/
and select Category: Integration.
For help see https://hacs.xyz/docs/faq/custom_repositories/