In Round 1 of FNF Vs FNAF, you can choose to fight these frightening characters in story mode or free play mode, but you must do the same thing to win, which is to play all of the notes of the song. You stick with the chart till the end of the song.
To do so, watch for the arrow icons to float and match on top of the BF, then press the same arrow keys.
Focus on not pressing the key too early, too late, or the wrong key, because if you do so too many times in a succession, you will lose the note and have to start over. Best wishes.