How to master the queen mate:

1. If possible, don't give a check until the move where you deliver checkmate.
2. Be very careful not to allow the opposing king to capture your queen.
3. Don't lose your queen. 
4. Avoid perpetual check and avoid repeating the same position multiple times.
5. Don't stalemate the opposing king.
6. Remember that the queen cannot checkmate the opposing king alone. 
7. Try to bring your queen in such a position where it is a knight's move away from the opposing king.
8. Once the opposing king is trapped on the side of the board, bring over your king so that the checkmate can be achieved.

The best way the win queen + king vs king endgames is not to give lots of checks.   

The queen can build a wall which the opposing king can never break through.