What's the etiquette for giving checkmate on board? I wonder what is considered to be a polite way of giving mate on board. Should one announce the checkmate? If so, should one press the clock after making the move, or stop it, or simply do nothing? 

Or should one press the clock silently as if it's a normal move and then wait until the opponent realizes (if they haven't yet) and shakes hands?
This is really a question for fairly low-rated players, who tend to play things out to the bitter end. If you are reasonably strong, the only reason your opponent does not resign earlier is that he thought you played such a good game that he allows you to execute the checkmate (I have done this), or thinks that the final position is beautiful, and saw it coming.

A very good example of the latter is this game between Peter Svidler and Magnus Carlsen at Grenke this past year (2019), where Peter allowed mate on the board. Note the BIG smiles on the faces of both players in this video.

In the end, it is just common sense and human decency: Just be humble, nice, and say "checkmate" politely, and shake hands. Treat the person the way you would want to be treated.

 [Event "Grenke Chess Classic 6th"]
 [Site "Karlsruhe/Baden Baden"]
 [Date "2019.04.28"]
 [Round "8"]
 [White "Svidler, Peter"]
 [Black "Carlsen, Magnus"]
 [Result "0-1"]
 [ECO "B30"]
 [WhiteElo "2735"]
 [BlackElo "2845"]
 [FEN ""]

 1. e4 c5 2. Nf3 Nc6 3. Nc3 e5 4. Bc4 Be7 5. d3 d6 6. Nd2 Nf6 7. Nf1 Nd7 8. Nd5 Nb6 9. Nxb6 axb6 10. c3 O-O 11. Ne3 Bg5 12. O-O Kh8 13. a3 f5 14. Nxf5 Bxc1 15. Rxc1 Bxf5 16. exf5 d5 17. Ba2 Rxf5 18. Qg4 Rf6 19. f4 exf4 20. Qg5 Qf8 21. Qxd5 Rd8 22. Qf3 Ne5 23. Qe4 Ng4 24. Rce1 Ne3 25. Rf2 Re8 26. Qxb7 g5 27. Rfe2 g4 28. Rf2 Qh6 29. Qc7 Ref8 30. h3 gxh3 31. g3 fxg3 32. Rxf6 h2+ 33. Kh1 g2# 0-1