Here are the steps: https://github.com/simulot/immich-go?tab=readme-ov-file#google-photos-best-practices

You first have to use Google Takeout to request all your data. They package up to 50GB in zip archives for download. Took 2 days until it was ready for me.

Then I created an API key in Immich, compiled the go command line tool and pointed it to my server, similar to this:

immich-go upload from-google-photos --server=http://your-ip:2283 --api-key=your-api-key --manage-raw-jpeg=StackCoverRaw /path/to/your/takeout-*.zip