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.
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.
To clip through the door, 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 the trick:
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.