If you know how to get a game pgn, then that is half the work right there. On a PC, simply right-click on the move to "add annotation" and then simply type away 
Personally, I put the game pgn into the analysis board and then type annotations from there, but some people prefer downloading etc. Just make sure to copy/save the updated pgn with annotations after you are done
Hello everyone I was wondering how annotate my games on chess.com?