I can try writing one here :)
After you see all the Pokémons in the Regional Dex(see, not catch), go to Sandgem Town, and talk to Prof. Rowan, then there will be a short Cutscene, where Prof. Oak visits, and you get the Poké Radar/Pokédar, and get your Pokédex upgraded to National mode.
After this, the two most notable changes are:
1) If you talk to Lucas(If you're a girl)/Dawn(boy)'s Sister in one of the houses in Sandgem Town, she will tell you that she saw a certain Pokémon on TV at a specific location, if you then go to that location, you will have a ~50-99% extra chance of encountering the Pokémon she mentioned
2) If you talk to Prof. Oak in Eterna City, you will be able to catch Zapdos, Moltres and Articuno as Runners(Platinum Only)
Now, go to any location with a fairly large patch of grass, i usually use the patch at Valley Windworks, as it's pretty easy both to chain in and to get to.
Now, Register the Pokédar to the (Y) Button; it's important to keep focused on this.
You should use a Repel(Super Repel is cheapest per step), as your chain will automatically break if you encounter anything else than what you are chaining.
The Pokétch App you recieved with the Pokédar will also be useful, as it will notify you if your chain breaks, and also show how long your chain it.
Now, STAND(no bikes) in the middle of the patch, and press (Y).
The music will now change to a sort of Jungle-ish theme, and exactly 5 patches of grass will start to shake, sometimes with little sparks on it.
Now you can start chaining.
Walk into any of the shaking patches of grass and MAKE SURE YOU REMEMBER HOW IT SHAKED!
The patches that shakes wilder, and with more sparks will usually contain rarer Pokémons, or Pokédar-specific Pokés; and a Purple-flashing patch will contain a Shiny(100%!)
KO the Pokémon that appeared, if you flee, you will break the chain.
If you did that, 5 patches will once again start shaking, enter the similarily shaking patch, and keep doing that.
Remember to avoid these patches:
1) The Patch right behind your character, you can't see these clearly, so you will usually fail if you enter this patch.
2) Patches in the border of the patch you are chaining in, especially corners.