You're offered a chance to visit Island 1 by Bill after beating the Cinnabar Gym. And I think the other offer is made after beating the Elite 4 by Bill if you did not accept it earlier (not sure about this part). Check the FAQs for more information regarding the Islands. --- "Hell, I'm surprised that you didn't ask 'should I breathe?' Life really is the most complicated of all games" - gbchaosmaster
If you've been there before, you can go there again through the Vermilion City Port.
If you haven't gone yet in your current save file, then beat Blaine and accept Bill's offer, or if you rejected his offer, go to the Cinnabar Pokemon Center and talk to Bill. --- If con is the opposite of pro, then what is the opposite of progress?