For the achievement 1CARU5, you need to fly with a leaf under all "stonebridges" in the game.
But there are only three different stonebridges where you need to fly under.

The first stonebridge is really easy to find, you only need to go to the first or second checkpoint and fly under the first (how can I say?) bridge.

The second bridge is the hardest to find and I think the only reason why you read this guide.
But when you see it you think that it's not to overlook and easy to find.
It's right next to the second checkpoint near the first starseed.
The second hard thing is, to fly under this bridge, but this is your problem not mine. ;P

The last bridge is very easy to find and it's easy to fly under it.
It's right over the third checkpoint and really not to overlook.
Try to go on the bridge, then jump of, turn around and then fly under the bridge.

I hope the guide has helped you and that you found all bridges.
This was my first written guide and I'm german, so please don't be too hard to me. :D
And the last thing: have fun with hunting other achievements!!!