Begin by understanding that there are two different processes. The first one to think about is the formal performance evaluation process that your company does.
As a supervisor you have to follow the procedures and forms that your company has prescribed. You need to conform to the corporate culture. That sets certain boundaries for the formal performance appraisal process, but it doesn't change your objectives for the process.
No matter what the process, the forms or the frequency of appraisal, there should be no surprises at this point. Your subordinate should not be surprised by what you say about him/her. You should not be surprised by anything your subordinate says.
To keep the formal corporate process surprise-free, you have to do a good job of performance evaluation all the rest of the time. The day to day work of performance evaluation is what supervision is all about. It's the job.
Most performance evaluation is short and informal. The supervisor mentions to a subordinate that their reject rate is up, or that handling a report differently will be more effective. This stuff happens every day in the cracks in the system. It never makes its way to paper because it doesn't need to. About 80 percent of the time, that's all you need.
If you're a supervisor, you should use every contact with a subordinate to counsel, correct, train, and encourage. Most of the time you can do that informally. Sometimes you have to start documenting.
Before you do that, though, you should tell your subordinate that you're moving to the formal stage. "John, I noticed that your scrap rate was up again. If that continues we're going to have to talk about it and I'll need to document our discussions."
The fact is that you can document positive things without letting folks know, but they'll feel betrayed if you suddenly start documenting things that reflect badly on them without letting them know. This is true even if they "should" know.
You only need this kind of transitional comment in about a fifth of your situations. In well over half of those, the threat of formal documentation is all that's needed for behavior or performance to change.
If behavior or performance doesn't improve, you need to keep meeting with your subordinate. You should document every meeting.
The documents, positive and negative, are what form the basis for the formal evaluation. I suggest that supervisors make a practice of catching people doing things right so there's plenty of that stuff in the file for the folks who are performing well most of the time.
Here are a couple of other bits of advice I give to my audiences.
Follow the Dinosaur Principle. Problems are like dinosaurs, they're way easier to deal with when they're small.
Manage behavior and performance, nothing else. Behavior is what someone says or does. Performance is the results of their work according to a pre-defined standard.