The first skip that is playable in runs is 6 skip. Sometimes you may die to 6 skip, but if you don't do it the time won't be nearly as good. To do 6 skip, hold space bar while aiming your camera towards the portal at the end of the level. Even if you die on the third jump on the death plane, sometimes one of your body parts will touch the portal, which will still send you to level 7.
The next skip that is playable in runs is 10 skip. What speed runners used to do was just jump on the death plane to the slope at the end, but the developers raised the death plane. So now, what we do is jump onto the death plane to the nearest tree (in the direction of the slope near the end), get a high jump to the next tree by running up the tree first to gain height, and then getting a high jump onto the death plane and then onto the slope near the end. It's not that consistent, but it can save some time if done quickly.
The next skip that is played in runs is 17 skip. To do 17 skip, jump onto the largest pyramid from the third or fourth platform, run until the end of it, turn left, run a bit, and then turn right and jump onto the platform that will be there. On that platform, there is a slope that you need to climb up. Your character gets stuck sometimes, so try to shake your camera a bit when you are running up it.
The next skip that people use is 19 skip. For 19 skip, you have to jump to the large volcano from the lower part of the third platform. Then, make your way up the volcano. It makes it easier if you vigorously shake your camera from right to left. Keep going forward until you reach the piece of lava that is straight ahead. Then, turn left a bit and run straight into the piece of lava in front of you. This lava piece will act as a slope and give you the height you need to get to the other side of the map. You must do this skip very quick to save time.
There is a 23 skip, but it's probably not worth mentioning because it's definetely not playable in runs and is very inconsistent. Besides 23, the last skip to do is 27. This skip is tricky to do, but is worth learning if you want to get good runs. To do 27 skip, get to the fourth ice cream sandwich, and then turn in the direction of the candy cane that is nearby. Jump onto the death plane and onto the candy cane. Then, turn right a tiny bit and jump onto the big pink island. Run over and down it until you are near another candy cane. Jump onto the candy cane, and then jump onto the ice cream sandwich that is in front of you.
The levels that I didn't mention in this guide either don't have skips or nobody has discovered a skip for the levels yet. With that said, I hope this helps people!
