Go to Skytown. Put your Portal out in the air, somewhere. Then fall off the edge. As soon as you start falling click your “Go home” button (so have it ready) Then when you get home, go back out your door or exit sign depending on what apt you have. You will be back in Skytown, but you can now walk on air! Also if you head down to the Juguar Temple it will work there too. It makes collecting the crystals MUCH easier. You will be stuck like this however, until you log out. So if you want to go to the Catacombs or mines, you will have to log out and back in.
NOTE: I tried it once without using my portal. I just fell hit my home button…. and so on. and it worked. I haven’t completely experimented with it. So maybe after the first time you put down your portal you don’t need to do that part again? Anyone who knows more, feel free to leave your knowledge in the comments 🙂
UPDATE: I just tried it without using my portal at all. And it worked. So maybe that is a pointless thing to do or maybe you have to do that the VERY first time you try the giltch and never again.. Someone do me a favor. If you are doing this for the first time. Try to do it with out your portal and tell me if it works?? THX