wot Al says.
Lots depend on the circumstances but the basics are as above. Meet with the person. State issues, ask if they need training or whatever establish if there are reasons for underperformance, set reasonable targets
Review meeting, targets not met, Warning, reset targets,
review – targets not met, final warning, reset targets,
review targets not met, sacked.
You need to be fair and reasonable and to take all necessary steps to help the employee with the underperformance =- training if required that sort of thing.
you also need professional advice