0 0 0It's really simple. Generate the smallest world you can, add soil everywhere, and then spread the jungle biome with jungle seeds. You can accelerate the time so it spreads everywhere. Then while holding shift spawn monkeys until you get the achievement.
0 0 0This achievement is extremely finicky. You need to start from a blank slate, fill the entire world with soil, then add jungle seeds. Once the entire square is jungle, you can start adding Monkeys to the world.