The advice depends on your device & app that your using, here's a link of previously asked/ answered questions: https://www.chess.com/forum/search?keyword=How+to+annotate
99% of the time I use my mobile phone via the web.
I copy the PGN into clipboard, then paste into Memo, then add/ edit any notes using these brackets {annotation notes}.
If anyone has a minute to explain how I can annotate a game and post it like I've seen other members do here in the forums, with side variations and whatnot, I'd appreciate it. I just want to discuss one of my friend's games that was played here on chess.com by private messaging him my notes. Thank you