The Smogon thread does a horrible job at explaining it. I watched a youtube video that did an okay job of explaining how to RNG (it's quite outdated by now). But a lot of the RNGing aspects I figured out by myself.
Anyway, before you're going to RNG you'll need to do some preparations.
==PREREQUISITES==
- Get EonTimer. The DS clock is a bitch to watch plus EonTimer knows exactly when you need to boot up your game
- Get yourself a pokémon that knows how to use Sweet Scent.
- Save beforehand in an area without NPCs moving around in random patterns (NPCs that stand still are fine). Victory Road is an excellent place. Save near the entrance where your 'rival' is.
==CALIBRATION==
Before you can RNG properly you'll need to know what Timer0 is more common for your game. Legit copies usually fluctuate between two different Timer0s (one is much more common than the other, and if you can consistently start up your game at the right time you're bound to always get the same Timer0)
If you play the game on a flash cart the Timer0 will fluctuate much more.
Start up RNG Reporter, click on the 5th Gen Tools tab and select Find DS parameters. Select your game version, your DS type, and punch in your DS MAC address (the Smogon thread explains how to find your MAC address)
Now, since we need to figure out our Timer0 it doesn't really matter when we start up our game. Just think of a suitable date and time, for example May 11th 3:00:25 PM. Start up EonTimer and type 25 in the 'Target seconds' field. Start up your DS, and adjust the date, then adjust the time. When you're about to confirm your time, make sure you press A on your DS and click start on EonTimer at the same time! Your DS will shut down to save the changes. Start it up again and wait until the timer on EonTimer hits zero. When it does, press A as soon as you can and make sure you immediately release the A button afterwards. DON'T PRESS ANY BUTTON UNTIL THE NINTENDO LOGO APPEARS
Phew, we're getting there. Load your game, DON'T activate your C-Gear. Open up the menu and use Sweet Scent. Catch the first pokémon you encounter, calculate its IVs (RNG Reporter can do this for you). Hit search and check if the actual seconds matches (in this case that would be 25 seconds if you used my example).
If the actual seconds correspond with the seconds you wanted, write down the Timer0 and copy the Seed value.
If the actual seconds don't correspond, try again (this means adjusting the DS clock again, etc.)
Personally I'd recommend re-calibrating anyway, whether you hit your time or not, to test yourself if you can consistently start the game at the right moment and whether your Timer0 will fluctuate or not.
Try to do the steps above first, then we'll get to proper RNGing.