Start the NetBackup Administration Console.  (see relevant doc)

In the menu tree on the left-hand side of the NetBackup Administration
Console, click Policies.  The Policiies window will appear  in the 
right-hand side of the NetBackup Administration Console window.

Click the name of the policy for which you wish to run a backup, then
right click on the name of the policy and select Manual Backup from
the pop-up menu.  The Manual Backup window will appear.

Click a schedule to select it, and click on the clients you wish to
back up to select them.  Press Ok when you are done.  The Backup
Policy Management dialog window will appear, stating that the manual
backup for the policy you've selected has been started.  Click Ok.