Your feedback will be most useful to the employee when it is:
Specific and descriptive, not evaluative. Effective feedback describes specifically what the person did rather than make a judgment or broad generalization.
Aimed at behavior under the employee’s control. Every action people take may be under their control, but some are more difficult, perhaps impossible, to change. Physical or personality characteristics are good examples.
Well-timed. Receiving feedback about last month’s behavior is like putting a cast on an arm broken the month before. Immediate feedback is best. Feedback should be delayed only to avoid embarrassing an employee before coworkers or to get more information.
Constructive. The manager should be seen as helping, not as attacking. Toward that end, your feedback should show the employee how performance could have been better.