How could someone get plastic surgery done without anyone knowing?

Yes. A carefully performed procedure does not have to draw attention to itself.
Depends on issue. This depends on the amount of change that is done. Also, some procedures are better hidden than others. Most often subtle changes can be made and not shout that you had something done. Best to see a board certified plastic surgeon.
Planning. Most well planned procedures are easily kept from others knowing. Plan the procedure carefully with our plastic surgeon to determine the amount of time necessary to recover.
Careful what u ask 4. You should be careful and precise about your requests. I have many patients who similarly ask for a discreet surgery (small breast implant for example) but are markedly disappointed when friends are not aware of the change. I think you are asking for subtle but discreet improvements and, of course, this is possible but not always 100% predictable. Discuss with a board certified plastic surgeon.