King, bishop, and knight versus a lone king is the trickiest of the basic mates — many strong players have to revise it. It's worth knowing because it can win an otherwise drawn game.
The mate is only possible in a corner the same colour as your bishop. So if you have a light-squared bishop, you must drive the enemy king to a light corner (a8 or h1). Aiming for the wrong corner is the classic mistake.
Use king, bishop, and knight together to force the lone king to any edge, the same way you would with other mates.
When the king flees toward the wrong (bishop-opposite) corner, the knight performs a zig-zag 'W' path to herd it along the edge to the correct corner. This is the part people forget — the knight does the steering.
With the king in the correct corner, the bishop and knight combine with your king to deliver mate. Keep an eye on the 50-move rule: from most positions it takes around 30 moves, so don't dawdle.
If it feels hard, that's normal — drill it a few times until the W-manoeuvre clicks.