This may get outdated when more fast strats get found. But for right now here is the guide for a good run. I will be skipping levels that have no methods of going faster.

Level 1: After collecting the key, jump onto the counter and over or on the creator's head to skip going around.

Level 2: Immediately after you enter the door, move toward the level, not the checkpoint. The checkpoint's hitbox is very large and you will easily touch it.

Level 3: Skip the 1st denture and jump to the second.

Level 5: Don't switch back to back across the checkers. Jump over the first red one, the next red one, then jump to the right and jump over the blue one and into the vent. This is pretty difficult so it will take some practice.

Level 6: Do the same thing as level 2. Go toward the level after you exit the vent, not the checkpoint.

Level 7: After entering, jump to the small middle square and across to the other one. Don't bother with the big squares.

Level 9: When entering the room, hit the button and jump up onto the drawers instead of going around.

Level 10: Jump past the denture to the far left and then alternate back and forth through the dentures close to the window. After that, jump past the denture on the corner wall. Yet again, this is something that will take practice.

Level 11: As soon as the screen starts fading start walking to the left. When the screen is fully black, wait a tiny bit and jump while still holding left and you will end up in the vent. When you hit the button in the room, reset your character and you will spawn in the other room where you can easily walk across.

Level 12: Jump to the platform to the right with a tight jump and jump back to the pipe ahead.

Levels 13 and 14: When you enter the door, turn around and move forward. When you get good enough, you can time it to where you can jump to the first platform while the screen is still faded.

Level 15: Do the same thing as Level 7.

Levels 20 and 21: As soon as you enter the mouth, hold right. You don't have to do any tricks this will just make it so you can progress further faster.

Level 24: Go in between the bones and turn on shift lock. Angle your camera to the right toward the bones to the right and hold the jump button while holding W and A. You will scale the ladder much faster.

Level 26: Do the similar thing as Level 14 but jump a tiny bit earlier. After you do all the jumps, jump up the fence and over the foliage on the left and touch the checkpoint to end the run.

This altogether may result in a 3:1X if you do it perfectly.