Go to Sandgem town and talk to Dawn/Lucas' sister in their house and she'll tell you what pokemon are swarming and where. Then all you have to do is head to that location and go into the grass. The swarming pokemon will have a fairly high chance of appearing in any grass on that route.
What pokemon is chosen for a given day's swarm is random, though each swarming pokemon will only appear in a specific place and at specific levels (for example, Farfetch'd will always swarm on Route 221 and always at level 28. Beldum will only appear on Route 228 between levels 51 and 54)
You can only have one active swarm per day, and it will change at midnight according to your DS' system clock. If you want to change the swarm without waiting, set your clock to a couple minutes before midnight, start the game, and wait for the time to pass before talking to teh sister again. If youtry to change the day or set it right to midnight or beyond, nothing will happen.