Two bishops and a king can force checkmate against a lone king. It's harder than the queen or rook mates because the bishops need your king's help and the mate only happens in a corner.
Placed on adjacent diagonals, the two bishops form a barrier the enemy king cannot cross. Advance the wall one diagonal at a time to shrink the king's space.
The bishops alone can't deliver mate — your king must come forward to take away escape squares. Coordinate king and bishops to herd the enemy king toward a corner.
Unlike the queen, two bishops can only mate a king in a corner (any corner, since the bishops cover both colours). Drive the king there, then deliver mate with the bishops on adjacent diagonals and your king guarding the escape.
This mate takes some moves, but it's well within the 50-move limit from any position. Practice it so you don't accidentally stalemate or shuffle aimlessly.