It is important to set out the grievance procedure clearly, and ensure that it is universally available.
Make sure that employees know who to approach with any grievance. Typically, a grievance is initially taken to the immediate supervisor. The grievance can then, if necessary, be taken further by putting it in writing to a more senior manager who must arrange a face-to-face meeting to discuss it. If that fails to resolve the problem, a final decision is taken at the highest level.
Set a deadline (say seven days) for responding to complaints at each stage, but allow yourself some flexibility. You must allow (and should encourage) employees to bring a colleague or union representative (who may put forward their views) to any meetings at which grievances are being discussed. That said grievances should be kept as confidential as possible.