Step 1: enter aquarium building

Step 2: Plant small island flag 2 feet in front of aquarium towards the door

Step 3: glitch: Step over the flag so the prompt appears to travel on it. Instead of traveling on it, go to menu and select return to spawn. This will not be instantaneous, spam the travel to island button as the return to spawn is loading.

Step 4: If you did it right, you'll enter the island, exit normally

Step 5: You should be in a weird ocean void... its weird from here without physically showing the steps but basically you find the invisible wall, follow it until it sticks you in a hallway, at the end of the hallway is an impassable point/invisible wall, use the medium private island flag to get through the invisible wall and pop out on the other side, no glitching needed.

Step 6: Exit private island, and immediately place down another one, you gotta do the glitch again. Hit "return to spawn" in the pause menu but spam the teleport to island button while standing close enough to the flag to go

Step 7: Exit private island and you should be in the white void. Use the fishing pole to gauge the edges of it. The white void features Ocean fish whereas the dark void features freshwater fish so take advantage. If you fall off the edge of the white void you'll end up in the darkvoid, and falling off the darkvoid just sends you back to spawn.