For comparison, here is the procedure at my work:
- Ask boss. Get permission. (spoken)
- Formally apply for leave using a computerized timekeeping system.
- HR sees this application, asks boss if it is OK. Hopefully they say yes. For trusted employees, don't bother asking boss.
- Application is formally approved. Note that this might happen after the actual leave if you apply the last day before. But that is fine.
- First day after leave, tell the timekeeping system that "Yes, I was absent." (This is still fine if the leave is approved later, as long as the application was entered before)
Now, are you sure your employer doesn't have a similar system and you skipped some steps out of ignorance?
You have been given a written warning not to do this again. Ask HR politely exactly what you did wrong. In other words, ask what the proper procedure is to avoid this problem in the future.
Be calm, be polite and listen to what they say.
In a comment you seem to be saying that your boss didn't do the proper paper work. If that turns out to be true, ask HR politely how you should best insure that your boss does things correctly.
Be calm, be polite and listen to what they say.
A warning is just a warning and not a punishment in itself. As long as you do not repeat your offense things should work out.
You might say at this point, "But it's not fair!" This is true, but the worlds is not a fair place. If your employer turns out to be even less fair than average, you might want to look for other work, but this single case doesn't sound like worse than average to me.