You need to enter the Fishing Store near the spawn (or aquarium, it doesn't matter much)

The second thing you need to do is put the Flag Prop at nearest the wall as it's possible
It must be outside the wall

Just teleport to the flag outside the wall
If you can't try to put the flag again

After teleport to the flag you will be on your island
Leave using flag

and boom you are in the void
You can fishing here, chatting with your friends etc.

There is also a chance that a portal/pit/hole leading to The Void will spawn
The Void Portal will appear on average every 67 minutes (0.25% chance every 10 seconds) and will last for 10 minutes before disappearing. (source from webfishing wiki)
Appearance of portal and a map of where they may appear:




I also wanted to thank one user for information about these portals,
it made me to do a bigger research :)

Just teleport to spawn
(esc + return to spawn)

You can also just go until you fell off

and that's all
I have hope my guide was helpfull :)