There is a small hole in the geometry on the sides of the bottom set of stairs leading up to Ankoku Temple in Ryoshima. This makes it possible to clip through with the correct movement. This location is conveniently close enough to the map's origin point that it is possible to interior clip into the Ryoshima Dojo. This saves time when traveling to the dojo to obtain double jump (Holy Eagle) after Blooming the Ryoshima Sapling.
It is relatively easy to clip through in such a way that Ammy slips straight off the floor into the void. When this happens, it is impossible to reach the dojo interior without a KT to regain horizontal momentum. This type of clip can be more consistent, so it is favoured during NG+ runs.
By clipping at a more precise location with a more perpendicular angle, it is possible to clip under the stairs but immediately land on a small ledge that extends under the stairs out-of-bounds. From here, one can do a Air Ground Tackle jump (jumping low to avoid clipping up through the stairs) to get enough momentum to reach the dojo interior. This is the method required in NG runs. See https://youtu.be/eY11GDCUXc8.
Because the dojo interior is relatively small, you should use the overlay map (hold L2) to line up the top of the nose of Ammy's icon with the bottom of the Ankoku Temple icon, then release the analogue stick to have Ammy fall straight down into the dojo. See https://i.imgur.com/ToOb4pu.png.
If you go too far North, Ammy will land in completely black room behind the door behind Onigiri-sensei. It is impossible to escape from this room, making it a softlock. It is also possible to land in a room underneath the main dojo room, which is also a softlock unless you can KT to clip up through the floor.
It is also possible to reach the dojo without going out-of-bounds and without Water Lily by performing a Corner Jump into a wall jump at the North side of the dojo's island.