In Aliases you're allowed to add your custom domains to create all of your personalized aliases. If you've ever purchased a domain in the past and managed DNS this will be simple.
If you've never done this before, fear not it's really very easy!
In Aliases, create a new domain, all that's required is the domain name itself.

After you create your new domain, you’ll see a message with 2 DNS records (MX Record and two TXT Records)

TIP: you can click to copy the names and values
Login to your domain registrar and use the DNS manager to add these 2 records from your domain:
<aside> 💡 Tip: Make sure to include the last period at the end of the MX record!
</aside>
Here's an example from Google Domains

First add the MX record

Next add a TXT record

Make sure to add the Host name for your 2nd TXT record