And now, to the Props menu (again, from the dropdown list on the left). Click the “...” next to Blueprint, then select the Evergreen tileset, the Trees category and, say, Pine06_Big from Prop. You can now paint bushes onto your island - the radius and intensity sliders on the top right will alter how big an area you paint at once and how densely forested it is. After that, go and add some other props - rocks, fog, whatever takes your fancy.