When dealing with performance issues, it's essential to be objective. By the time you feel the need to step in, you've probably given plenty of informal feedback already. Your employee should have a clear idea about what he or she is doing poorly.
Documenting this feedback may be important – and you should also know whether the employee had enough time to improve. Of course, the feedback must be unbiased and objective. So you might need other managers or colleagues to support you.