Visit the StellarID.io site and set up an account:

https://stellarid.io/

Add a domain by clicking the green "Add a new domain" button. Input a domain you own in the text box that appears and submit.

Add your first Stellar address for testing purposes. Click the blue "Edit" button and submit your desired username and the associated Stellar account id.

Add a stellar.toml file to your site at https://yourdomain.com/.well-known/stellar.toml that points to "https://stellarid.io/federation/".

It's important that you use your naked domain for the toml file URL - Stellar clients will not look for it under your www subdomain.

Modify your stellar.toml file/page to enable CORS - cross-origin resource sharing.

You can do this by adding "Access-Control-Allow-Origin: *" to the HTTP header.

Test that your set up is working properly using StellarID's testing tool at the bottom of this page:

https://stellarid.io/domain/lumenauts.com/setup/