Note: If you're going to do this glitch and think you've done it incorrectly, RESET your game or else you'll lose your saved data. This only works for D/P, not Platinum. This glitch also works for Shaymin, too. Also, this will take forever to read so don't read it if you don't wanna die of boredum. :P
Other crap: Sometimes trying this glitch freezes your game, just turn off your DS and try again. Sometimes you make a void, but can't get into it, so turn off again and try again. DO NOT DO THIS WHILE READING THIS. THERE MIGHT BE SOMETHING I SAID LATER YOU DIDN'T KNOW, READ THE WHOLE THING UNTIL YOU KNOW HOW. ALSO, the first one I'm talking about getting is Darkrai, so if you want to get Shaymin, read Step 2, then see below.
What to have (MUST HAVE):
Poketch Marking Map and/or Berry Map, Step Counter,
Pokeballs (I would recommend Ultras).
Pokemon with Fly or Dig (you must have Fly, actually, also this is for if your going for Shaymin)
You do not have to have the Nat. Dex or 8 Badges. You can do this glitch as soon as you got everything you need. You have to have used the Explorer Kit AT LEAST once before using this glitch.
Tweaking is moving the pad too quick in order to make the game hard to keep up, making the DS glitch. Ok, go to Jubilife City. I would show you how to get into the void, but it's going to be too hard for me to explain, so watch this video to show you how you go and in which direction: http://www.youtube.com/watchv=LdRHx4ql9Dg
Step 2: Go 20 steps west, 10 steps north, 5 steps east, 3 steps south. After this, go to your menu then go to Pokedex, Pokemon, Bag, ID Card, or Option. Then exit. Press down. You should be in a void outside the inside of the (I think it's the TV Station or Poketch Company) Save. Rest your Step Counter. Go to the right side of the wall in the void, 1 step away and go up until you reach Jubilife. It'll say you are in Floaroma Meadow, Veilstone City, etc. This will take about 430 steps. When you reach Jubilife City, STOP. Go back to the first step you took in Jubilife City, and stay there. Register your explore kit (when you press Y you can use it without going to the Bag section). Use it. It should say "A communication error has occured. You will be returned to the title screen. Please press the A button." After you're sent back, go to your file. Use your bike, but don't register it. Reset your Step Counter. Start riding down. Head south until there's an invisible wall blocking your way. Go up 2 steps, head west until you reach Jubilife City. Half way into Jubilife, then south. There might be a wall, so try different spaces to get through. Then, get your Marking/Berry App and see where you are, you should be in the ocean near New/Fullmoon Island. Go west, Reset your counter app. Go west again for 63.500 steps. This should take 70 minutes. Yep, 1 hour and 10 minutes. If you want, get something heavy (not too heavy that it'll break your DS) and put it on the left directional button. That way, it'll just go on its own. (Or use tape to hold it down. :P) You will run into a wall. From there go up, west, up, west until you find a way to go past the wall. And keep moving west, you'll reach a place people call "Fake Sinnoh".
Step 3. DON'T GO ON YOUR MENU OR ELSE YOU WILL BE STUCK IN THE VOID FOREVER. Yay! You are in the ocean, near Shaymin's place. 5 steps above Shaymin's place, go west 760 steps, south 60 steps. All these I mean "about". You go faster to New Moon Island on the top than going from the bottom. Then you reach Fake Sinnoh. Find New Moon Island. Try not to save with the kit here. Head north. This take 65,000 steps. You look like your in the island, but the Jubilife music plays. Now, save with your kit (Y). You will be taken back to the menu, then go back to your file. Now you go to your Menu, and go to any section I mention in the last 30 minutes. You're in NewMoon Island. Now you can save. Go and battle Darkrai. Yeah, that's it. But if you were hoping to get Shaymin, continue reading.
From the part where I said to take out your bike in Jubilife void and in the first step in Jubilife, stay there in Jubilife void. Also, this is where you MUST HAVE Fly/Teleport. From there, reset Poketch Step Counter, and head east for about 870 steps. Then head north, 64,000 steps, you'll be a Fake Sinnoh below Sunnyshore. From there go to Sea Break Path. The graphics have been screwed up in Fake Sinnoh, so the path is white. Ride inside the white path until you arrive at a Jubilife area. Press Y to use the kit WITHOUT popping up the menu. When you reboot you should be in the real Seabreak Path. Pop up the menu to reload the graphics and head over to Shaymin.
Of course, this took forever so I might have some things wrong. If you think I have let me know.