Where to put schema when using custom collection?

[Whoever]

See below. The WSDL looks good, the result is OK but not Schema. Where and
how should I put it in. Hopfully not manually. Thanks.

public class aUser {
public string UserID { get {...} set {...}
public string UserName {...}
}

public class UserList : System.Collection.CollectionBase {
public void Add(aUser user) {...}
public aUser this[int Index] {...}
}

A web service:

[WebMethod]
[return: XmlElement("UserList", typeof(UserList))]
public UserList getUserList(string search) {
UserList ul = new UserList();
return ul;
}