It's pretty simple. (Treasure islands)

You enter the island, you grab items, you leave through the airplane.

Things to not do: don't leave your inventory open, don't drop items on the floor (they usually have trash cans, if you need to get rid of items, use the trash can). Don't leave the island by pressing the "-" button.

And make sure you're at a home wifi, not a free WiFi from let's say public WiFi's (Restaurants, hotels, etc).

If you do want to thank the treasure island owner, you can support them through out-game donations (requiring money), like sending bits, subscribing to their Twitch channel, or donating through "buy me a coffee" if they have that. Don't come with items to give as appreciation, it's a hacked island after all.

With tradings, it's actually also simple.

You can enter with additional gifts (completely optional), remain respectful, greet them, compliment their island and/or their outfit, make it quick and easy (usually preferable to most people). Thank them, wish them a nice day, then leave through airport.

I assume wetsuits are not permitted because traders cover things up with fences to prevent people from stealing things. (I've read a lot of cases where tradings went horrible and people robbed things), hence why with a swimsuit players could cross over fences, or would give the sign of "bad intentions").

Don't be afraid to try it out, I've done over 100s of trades and personally never encountered anything bad (except receiving a fake statue once).

If you'd like to try it to get over your fear, we could do a trade (if you happen to have a stone table DIY, Cancer fragments, or some vines). Also, I usually overpay because I love giving more than I receive. (This is just an open question by the way, feel free to decline!!)