Use either the AT scheduler (need to go to the command prompt to do this) or the Task Scheduler (this will be under My Computer, but will only exist if you have Internet Explorer installed on the server).
Task Scheduler has a normal Windows type interface, so you should be able to work it out.
AT scheduler requires a bit of DOS knowlege. If you have Task Scheduler, AT scheduler will not work.
Check that which ever schedular is enabled in the services.
With both you will first need to set up a batch file which runs NTBackup, then schedule this batch file to run at your required time.