Early Master Sword is a sequence break in TWWHD that allows one to watch the post-Helmaroc cutscene before having obtained the Master Sword in Hyrule 1. That cutscene equips Link with the Master Sword and the Hero's Shield, and gives him access to Hyrule 2. There is currently no known way to defeat Phantom Ganon and obtain the Skull Hammer without the Master Sword, and so this sequence break can be broken down into two parts: entering the Helmaroc fight without hitting the peg that opens the door with the hammer, and Helmaroc Skip.
Note that doing this sequence break makes breaking the Barrier in Hyrule impossible, since watching the Master Sword cutscene in Hyrule 1 is necessary for the cutscene of the electrical barrier disappearing in Hyrule 3 to play, which is the final trigger that allows the Barrier to be broken.
Because the courtyard cutscene in Hyrule 2 causes a crash if you haven't unlocked KoRL, you MUST beat FF1 before doing this sequence break. If you can find a way to skip the courtyard cutscene without having to unlock KoRL (for exemple by obtaining the triforce early before unlocking KoRL), you would be able to skip FF1 for this sequence break, but there is no known way yet.
One way of doing Early Master Sword in TWWHD is by spawning on layer 8 in the overworld. This can be done by skipping the gossip stone on your first visit to Forsaken Fortress and then then coming back a second time, using the pirate ship, to reclaim the gossip stone and beat Forsaken Fortress a second time. Doing this causes Link to spawn on layer 8 in the overworld which is a result of the game expecting the cutscene where Link and the King of Red Lions talk for the first time. This cutscene does not play but the game still sets the layer for the cutscene anyway. On layer 8, the door blocking entrance to the Helmaroc Boss Room in Forsaken Fortress does not exist.
This method can be done faster by performing Outset Escape because the door will always be unloaded on the layer that follows the Forsaken Fortress 1 cutscene. With Outset Escape, it is possible to get the grappling hook before beating FF1, which allows this sequence break to be performed in only one FF1 trip.
If you don't manipulate the layers and go to Forsaken Fortress 2, there will be a door blocking the entrance to the Helmaroc fight. With an item slide, it is possible to reach the door without triggering the Phantom Ganon fight.
Several ways to clip through the door have been developped through the years. The first method is the most consistent while the second one was the first setup found.
First of all, make sure you have gyroscope turned off. For this setup, first go into the far left corner by the left side of the door frame and turn so that link is facing the door frame. Take out the grappling hook and aim the red dot towards the center of the door frame. Any angle between 4850 and 4700 should work. Then Item Slide forward towards the corner you want to clip through, and press and hold ZL to stop charging speed once you have more than you need (see below for more info on how to get the speed). Link’s speed should be around 3000 for best results. For the remainder of the trick do not let go of ZL. Next hit left on your control stick. This will change your speed to be a positive value, which is required for the clip to work. Now hold up on the control stick and while doing so adjust your camera very slowly until Link's animation shows him running forward instead of shuffling his feet. This will make Link face the same direction but will change the direction he is moving so that when he clips through he will get stopped by the little door that is underneath the ground and land in the correct position.
Then wait while continuing holding ZL and up on the control stick. When you reach around 2400 speed you will clip through and link will jump (without holding up link would not jump) and land inside the door. Sometimes link will simply grab the door and climb up which will still lead to a successful clip.
To get the speed consistently, you can look up for when the camera sinks into the ground slightly. This is called a weirdshot. It happens at around 2000 speed, and then the camera goes back up at around 2400 speed. As soon as the camera is back on the ground, wait one or two frame and then you should have the wanted amount of speed for this trick which makes it a good visual cue. Link has a small chance of clipping in place while charging speed. This usually leads to needing to restart the item slide because it is very rare that you had the correct angle to clip. Due to the exponential nature of item sliding speed growth you will pass over the clipping range 95% of the time depending on ESS position.
Notes: If you land at the stairs, that means you went too far right when clipping, which happens if you went too far right with your camera adjustment. If link begins to sink through the ground (this is purely visual and is known as a weird shot), that means you did not charge enough speed. Even if your angle is correct there is a less than a 10% chance that link will not go through the door frame and will simply jump in place. That means that link lost speed too quickly and clipped at too low of a speed. This is much more likely to happen (about a 50% chance) if link is in the moving sideways animation rather than running forward animation because link loses speed much faster when in this animation. Thus keep in mind that link will clip much faster when in the moving sideways animation thus giving you less time to set up the correct camera angle.
This method is a lot more inconsistent, since it requires pause buffering until Link clips to perform a jumpslash.
To clip through the door using this method, you will want an angle value between 22392 and 23087. This can be obtained by grabbing a ledge to get a consistent angle, letting go, and slashing a bomb vertically.
Then, you will want to enter a first person item and perform a sidehop to the left, making sure to release ZL in mid-air. This will make Link's next item slide shoot exactly to the right when pressing ZL, but still have negative speed, which is required for this trick to work. Make sure to not leave the first person item camera after doing this or the property will be removed.
Then, you will want to item slide towards the left side of the door and press ZL. The speed needed to clip being very precise, using a metronome is advised. When Link reaches -2850 speed, he will clip through the floor on the left side of the door. Link will then shoot to the right, and he will collide with a small door that is under the large door, which will stop Link in mid-air. From there, you can perform a jumpslash to gain extra height and hopefully land inside the door, and then enter the loading zone. Note that sometimes there is not even a single frame for the jumpslash to work.
Instead of a jumpslash, there is also a single frame where Link can perform a sidehop, and so this trick doesn't necessarely require a sword.
Here is an in-depth tutorial of this setup (this tutorial is slightly outdated, instead of holding straight left on your control stick, do the sidehop trick described above):
From the second highest platform on Forest Haven it is possible to perform an incredibly precise Item Slide aimed at the Forsaken Fortress top level loading zone and get behind the door before the collision of the island loads.