1. Pay the 10,000 down payment and talk to Nook. 2. The next day when you have your house, tell nook you want to upgrade. 3. Pay the 39,800 bells for that upgrade at the ATB in the Post Office, then talk to Nook. 4. The next day, Tortimer will be by the wharf/dock. Talk to him. 5. The next day, pay Kapp'n 1,000 bells and he'll take you to the island.
I'm not sure if the 1,000 bells needs to be paid every time you want to go to the island or just a one-time fee.
Opie & Anthony | Ron & Fez, XM 103/Sirius 206 XBL: Maxx the Slash | PSN: Maxx_the_Slash | 3DS: 3222-5637-7984 | Wii U: Maxx_the_Slash