Giving feedback on a performance deficiency is not an easy task but it is crucial to the resolution of that deficiency. It should follow the same structure than any normal feedback but should be complemented by:
a clear description of what are the expected standards of performance (i.e.: describe what good performance should look like)
Ideas of corrective actions & behaviours (i.e.: give specific examples of what could have been done differently and how it could have been done differently)