1)
How will they give honor to Hashem?
1.
Radak: With their mouths (verbally).
2.
Malbim: They will recognize the Chidush of the world, and Hashgachah.
2)
How will the islands tell His praise?
1.
Malbim: They will recognize His unity and spirituality, and that He overrides the Ma'arachah. This corresponds to "I will not give My honor to another, and My praise to idols" (8).