You can get the pokeradar by completing the Sinnoh pokedex. You have to go into Professor Rowan's lab and talk to Professor Oak, who will allow you into Pal Park. Then go to Pal Park and Prof Oak will give you a pokeradar.
Dude, this needs an awful lot of explaining.....but I'm willing to do it ^_^
Alright, first you need to make sure you have plenty of max/super repels because this is important.
What to do:
1. When you start the pokeradar you can run into any shaking patch of grass you like but make sure you use your max/super repel before you go running around in the grass. Remember to use another repel when your old one runs out.
2. When you're in a battle with the pokemon you want to chain, defeat it or catch it (I recommend defeating it or you might eventually run out of room in your PC box) but DO NOT run away from it or you'll break the chain.
3. When you're out of the battle there will be more shaking patches of grass. There will be a maximum of four patches shaking at one time. Look at them and remember where they are: this is where we come to the rules of chaining.
Rules of chaining
1. The next patch of grass you need to go into in order to keep the chain going needs to be the one that is the furthest away in terms of steps (in other words, the patch that takes the largest number of steps to get to).
2. The patch of grass that is the furthest away needs to be at least four steps away but no more than seven steps away.
3. Only choose a patch of grass if you see all four patches shaking. If you can't see them this might mean:
- you are too close to the edge of the big grassy area (I'll explain this later)
- your eyes weren't quick enough to see all four patches
4. Only choose a shaking patch of grass if it was the same type of shake as the first one you chose. That is either one that shakes mildly or one with white flashing in it, and Speedy Sonic told me there's another type of shake similar to the mild one that I didn't know about but I still don't get it because my eyes are useless...so sorry about that.
5. Don't go out of range of the pokeradar. This is when the patch of grass you reset the pokeradar in has gone off the screen because you ran too far out.
6. Don't go into a shaking patch of grass which doesn't have eight patches of grass surrounding it. See my explanation on page 5 of this thread for more details.
7. Don't go into a shaking patch of grass which is right next to another shaking patch of grass (same reason as point #6)
8. Don't use items such as your Bike or, correct me if I'm wrong, rare candies.
If you can't fulfill all of these points then walk 50 steps and reset the radar.
As I said in point #3, try not to go too close to the edge of your big grassy patch in your chosen route. Oh dear, this is going to be difficult to explain...um...I'll try anyway.
When you reset the radar in a patch of grass there are four possible "rings" in which the patches will appear. Ring 1 will be the ring of patches of grass around your patch of grass, ring 2 will be a bigger ring around ring 1, ring 3 will be a bigger ring around ring 2 and ring 4 will be a bigger ring around ring 3. Each patch will appear in a ring, so you can't have two or more patches of grass shaking in the same ring. I hope you know what I mean by "ring." Jesus, I don't think I explained that very well, I hope it makes sense :\
*strokes chin* I hope I haven't missed anything and more importantly, I hope I helped you.