The repair kits in Madhouse difficulty

So after a fair bit of searching I have found this guide. The guide is actually for obtaining the 'dirty coin' but it also happens to result in the 'true ending'. Using this guide I have managed to work out the minimum steps in takes to get the true ending:

1. Pull the hidden level in the fireplace to open the secret door
2. Enter the secret door and grab the fuse on the stool
3. Put the fuse in the fusebox (in the room where you start)
4. Go upstairs, activate the attic stairs, go up to the main room in he attic and grab the basement key next to the bed
5. Go to the basement to get the attic window key, at this point a Molded will spawn and you MUST get out of the basement without taking any damage
6. Return to the attic and escape out of the window (up the ladders) using the key

Essentially, you just need to escape without taking any damage and you will get the true ending.

TIP for step 5:

1. Grab the key then immediately run past the Molded (sometimes it will hit you, sometimes it will miss)
2. Immediately try to open the door to get the animation started
3. Turn around and get into a position where the hanging body bag is between you and the Molded. Kick it into the Molded so it gets knocked to the floor
4. Run to the door and keep trying it until you get through - you should get enough time before the Molded gets back up and attacks you.

It may take a few attempts to pull this off, but it is very doable once you get the hang of it.

According to this article you must first beat the game on any difficulty:

There are two ways to unlock this mode. The first is to have pre-ordered the game which grants a code that unlocks this mode from the very beginning. The second is to beat the game once on any difficulty mode.

This article (though from the same website) states the same thing.

...or by beating the game once on any difficulty.

I believe the previous entry in the series was also the same, and likely were many of the other Resident Evils.