This one will take some patience. You will need the entire map unlocked from side to side, then ensure every square has something built on it. If you have any gaps, you can use single tile items such as decorations to fill up the empty spaces. If you are having trouble finding your last space, you can select the decoration shop, select a 1x1 flower and move your mouse over each space of the land. When your cursor turns from RED to WHITE, that's a space you can build on.