White must drive black's king to a corner square of the same colour as the bishop moves on, e.g. white in this example. It is relatively straightforward to drive the black king to the edge of the board so we start from this position.
1.Nf7
Kf8
2.Kf6
Kg8
3.Bf5
Kf8
4.Bh7
driving the king out of the 'wrong' corner
4...Ke8
5.Ne5
Kd8
6.Ke6
Kc7
This is the hardest part to see over the board. Black's king seems to be escaping from the edge of the board but it is a short-lived escape.
7.Nd7
Kc6
8.Bd3
This move prevents the king getting away. See how effectively the bishop and knight combine here to box the king in.
8...Kc7
9.Be4
Restricting the black king further and forcing him to return to the edge of the board.
9...Kd8
10.Kd6
Ke8
11.Bg6+
Kd8
12.Bf7
A waiting move.
12...Kc8
13.Nc5
Kd8
14.Nb7+
Kc8
15.Kc6
Kb8
16.Kb6
Kc8
17.Be6+
Kb8
18.Nc5
Ka8
19.Bd7
Another waiting move. An immediate Na6 would be stalemate! [19.Na6
]
19...Kb8
20.Na6+
Ka8
21.Bc6#
*