The islands can only be reached by ferry.
From the south a ferry leaves from Ampana every few days. It's wise to check up on the schedule before you head for Ampana. The ferryride will take a couple of hours, depending on how many stops they make along the way.
From the north, about once a week, the ferry leaves from Gorontalo. It's best to check up on the situation/schedule in Manado. Expect around 12 hours on the ferry. You can rent small (and not so clean) cabins for a hefty fee on the ferry, but it can be worth it especially if you are 4 people as there are 4 small beds. A meal (rice + egg) should be included and can be obtained below the main deck near the engine...

Alternatively you can charter a speedboat to the islands but it will cost you.