When you start night 5 the first 20 seconds of Fredbear or Nightmare moving around are fake, he hasn't actually spawned in yet (only for night 5), but still move from door to door because both have a kill timer that stops you from idling in the bedroom. You'll die after 15 seconds of not moving. Once you hear him stop moving for about 3-5 seconds, he has chosen a door and silently moved forward.

Once he moves forward you will have 15 seconds on night 5 and 6, 10 seconds on night 7, and 8 seconds on night 8 to find the boss before he kills you. Run to the door he stopped moving at and flash down the hall to see if anyone's there, and if they are, shut the door till you hear running. The flashlight is not pointless as some will say because sometimes when I went to the door and flashed I saw he wasn't there, so I heard something wrong (I guess) and had to find him somewhere else. It's a good check to make sure he's there so you know for sure where he is. JUST TAP IT THOUGH, holding it will make him kill you instantly, so either tap it once or flash him by tapping it one frame over and over to make sure he's there as you may have got to the door too quickly and he hasn't shown up yet with his silent creep down the hall, so you can keep flashing for a few seconds to see if he shows up but I always use the flashlight at least once, it's good to know for sure where he is at all times; it's not required but it helps. (Also, if you are running out of time to find him just take a chance and close the door without flashing as it will waste time which - especially with Nightmare - is time you don't have. If you feel you don't have a second to waste, don't flash, just close the door and hope he was there)

And if you hear his laugh CHECK THE DOOR YOU LAST HEARD HIM RUNNING TO FIRST!!! The laugh means that he went to either the bed or the closet, in which case deal with him like you would the Freddles or Foxy, but the reason you check the door he ran to first is because he can do a fake laugh, and in my experience he does more fake laughs than real ones. So if you hear him laugh, check the door assuming it's fake and if he isn't there after your one frame flashes then check the bed, if he's not there check the closet, and if he isn't there either check the other door. If it still isn't there then you're probably dead as he was still moving and hadn't chosen a door yet and all your running back and forth muddled the noise, so either quit, shut off your volume, or check both doors again and hope you catch him. F2 returns you to the title menu in all ClickTeam games btw so you don't have to quit the actual game :)

Catch him at the doors and with his laugh and in 4-6 mins based on your plushtrap winnings you will have beaten Fredbear and Nightmare every time. But it's easy to make mistakes, and Nightmare is still really tough, good luck!

P.S. On night 5 you will be dealing with only Fredbear and on nights 6, 7, and 8 you will be dealing with the main cast of characters with Fredbear and Nightmare spawning respectively at 4 AM. Night's 5 and 6 are Fredbear and night's 7 and 8 are Nightmare.