Witnessing is a technique use during internal auditing that is applicable to many activities. Typically a laboratory would as a minimum include technical witnessing of personnel. You would decide which activities to witness, based on risk. Look at the scope of work and for a particular activity, e.g a test method, why you would decide what steps to audit. Take the process of receiving samples to releasing results as an example. If you have new personnel performing sample preparation, one Internal witnessing could be them demonstrating that step. If for example there have been nonconforming events raised due to problems with reagents, then plan to witness that activity.