The Login Manager is an easy to use tool. Simply double click the Login Manager node in the Enterprise Manager and the Login Manager will open.
The Login Manager
Selecting a login and using the Edit Login button will open the Login Properties dialog.
The Login Properties Dialog
If you create a new login, this dialog will appear as well - but empty of course.
Once you have created a login, you can't change every option available - for the meaning of the different options, please check the MS SQL Server documenation.
The Server Roles tab let's you add/delete the login from a fixed server role.
Login Properties - Server Roles
By checking or unchecking the server role name at the left part of the screen, you'll add or remove a login to or from that role. At the right, there's a list of actions that the role member can perform. The bottom part of the screen holds a quick description for the role, these descriptions come from MS SQL Server itself and cannot be changed.
Exitting this dialog by using the OK button will save the login information.