The idea is to chop trees leaving an acre of land.
Just try to put coins in the most number of hammer guys to chop threes from left to right until you get it.
Best to do on the second island since you will start from the beggining with some that came on the boat.
If you manage to do the amount of trees needed you'll get it. Keep in mind it should be done before the end of the 7th day to unlock it.
It should be around 25-30 but it depends on the size and lenght from the land where they were.