To set up a domain alias you should already have a primary domain. Other than that, two other things are needed:
- Another domain
- A DNS provider
Will assume that you already have "another domain" therefore the piece missing is the DNS provider. The best option for this is to use netlify itself (https://app.netlify.com/dns). Just add the new domain and point it to your app name.
Set up you alias in advance or it won't be included in the SSL certificate generation. Visit the following site for more info https://docs.netlify.com/domains-https/https-ssl/#domain-aliases.
Tip: You can renew the ceritificate in the site "Domain management" settings.