If you don’t know how to get the ID from a username, use the Get Users endpoint https://dev.twitch.tv/docs/api/reference/#get-users and specific their username as the login param. In the response will be their Twitch ID which you can use with that Get User Follows endpoint.
New twitch api is not super convient for gathering lots of information.
Like if a person has 300 followed, in order to return a full list of everyone live right now you would need to call the Follows api 3 times, then make 3 separate calls to streams in which you fill all the ID’s from the first call into the parameters. Then append the resulting json’s together to get your final list of live users a person follows.