First you must click on one of the domains on this page, once on the Vision.io website, you will access the profile of this domain. If this domain has the subdomains option activated, then you must press the «Mint subname» button in orange.
Once you have clicked «Mint subname» the box will appear for you to write the subdomain name you want to register. When you put your name, you will create a subname or subdomain with the structure yourname.domain.eth.
The subdomain will be yours forever with no renewal cost.
Revoke can unwrap
The subnames can be independently controlled to the ENS parent domain if you activated «Revoke can unwrap» when you configured the subname on Vision.io