1. Download the patch that is attached on the bottom of the post.
2. Apply the patch. The easiest way (but certainly not the only) is to use an emulator with "soft patching" (such as VisualBoyAdvance) and rename the patch so that it's the same name as the Pokémon Diamond ROM name (and place it in the same folder as the ROM), but with .ips at the end.
3. Go to the Tulunk Water Station. (Hey, I had to do it somewhere.)
4. Now exit on the left side (indicated by the box I drew). Don't do it on the right, or you'll just exit like normal.
5. You're at the Palan Antenna Tree! Don't go down, or you'll have to go all the way back (because the glitch makes the door one-way).
6. Enter the tree, and you're at Liz Island!
Now that you're here, you can play the game without the patch just fine. This SHOULD work without any problems. Please tell me if there's anything wrong! I only tested this on Diamond, but I'll try it on Power and Speed (though, I have never heard of the glitch happening in the Japanese games).
Oh, and by the way, yes, with this patch, it is possible to just skip to Liz Island really early in the game, but don't do it, because that's cheating. >_> Besides, you won't have that much luck anyway, because the Denjuu there will be at much higher levels than yours.