Select brush tool and click the island tile type.

Tile brush

Brush tool

With brush tool selected, click anywhere on the map to add island. Or, use​ right click to remove island. You can't add island on the black holes at the edges of the map. You can also add wood or hole tiles by selecting the appropriate brush type. Wood tiles are destructible, which means it can be shot at and destroyed.

Brush types

Tile brush types