After you do the Billionaire cheat, The 1st day of spring (doesn't matter what year) Thomas will say,"My what a large sum of money you have there." He'll say something else (I forget) and then the game will ask you:
-Buy
Don't Buy
You press buy.Thomas will say that the island costs $99,999,999. Then it'll come up a 2nd time.
-Buy
Don't Buy
Press buy again.
Then you will want a cottage on the island so you want to call Gotz the Woodcutter, and have him build a cottage for 20,000,000 g. You say go ahead. He will have it done about 2 or 3 days. He may come the 2nd day, but when you go there you may see him still working. But you can go inside, the only thing you'll have in the cottage will be a bed in the corner and a table in the center.

To go to the cottage/island, you have to go to the beach and there will be a log with a boat on the right hand side. you should face the boat and it'll say:
"You want to go to the Island"
You say "Yes"
Then when you wanna leave the island, go to the boat @ the dock and it'll ask:
"You wanna go back to the village"
You say "Yes"