Banjo-Tooie

This achievement can be gotten in Cloud Cuckooland.

Use the beak buster on the brown circular sections in the garden near the start. Two will contain seeds that you need to pick up.

Use the flight pad near Mumbo's skull and make your way around to two islands where you can plant the seeds. One is near the giant cheese wedge, the other is near Mr. Fit's 2nd location.

Then, find the real Mumbo's skull. There are two- one has a fake Jinjo on the lower level and one does not. The one with the fake Jinjo has the real Mumbo, the one without has a fake Mumbo boss fight. Take control of Mumbo (the real one) and make your way to a high exit from the central cave that leads outside to a purple racetrack. A Mumbo pad can be found on a ledge jutting from this purple path. Use the Mumbo pad to trigger a brief cutscene, after which the achievement will pop.

I have no idea what the other solution is going on about. I never planted the seeds for this achievement.

Edit:
Depending on which skull you enter first, will change where Mumbo spawns.

Go to Cloud Cuckooland, go into the red Mumbo hut (lower exit by warp pad in central cave). It has red around the border. Switch to Mumbo and go back into Central Cave. Go to the high exit by the big red ice wall. If you run straight out of Mumbo's hut, enter central cave, run up the skinny ledge. You'll see the yellow spots on the wall, you really can't miss it. Run past those yellow spots and along a skinny ledge. You'll end up on the purple race track. Use Mumbo's magic pad and he'll create a rainbow.

If you can't find the entrance I'm talking about to get to red Mumbo's skull. Collect the boots that allow you to climb the wall, run up the skinny ledge to the yellow feet, run up the wall and enter the door. To your right there are a few jumps you have to make and then down below is the red skull.

If the real Mumbo spawns in the Silver/Blue hut for you, it's the same concept as above except the ice around the door to get to the skull you need is BLUE instead of red.

that's all there is to it. no extra flying around to plant seeds.