Instructions
Finish Level 6-5 using the standard exit, not the hidden one. You will not be able to access Level 6-6 if you take the hidden exit in Level 6-5. Simply avoid climbing any beanstalks in Level 6-5 to avoid the secret exit. The standard exit lies to the far right on the level map.
Enter Level 6-6 and slide down the slope. This will take you to a dark cave. Jump onto the raft and step on the switch to start the raft moving. The raft will provide much needed light to help you through this level. The direction of the light depends on the way you tilt the Wii remote.
Avoid enemies while riding the raft through Level 6-6. Although there are platforms above the raft that you can explore, it is safest just to stay on the raft until you pass the section with about a dozen pipes poking down from above. Jump onto the next set of platforms that appear and hit the progress-marking flag in order to save your progress. Jump back onto the raft once you activate the flag.
Ride the raft until you come across the first fire-shooting Koopa. Avoid his fire, but stay on the raft until you cross underneath the Koopa's platform. Jump onto the next small platform, using the blue block as a stepping stone, then jump onto the thin platform above that. Continue to avoid any fireballs shot your way. The hidden red tube is located above this platform.
Enter the tube and make your way through the rest of the level. Beware the bats that attack from the ceiling in the next short -- but still dark -- map. This will open a path to a cannon that allows you to skip the rest of World 6 as well as all of World 7.