Focus on one stat. Here is an example of what you should do:
1 EV per Pokémon
+4 EVs for Power item
X2 for Pokerus
[(1+4)X2] = 10 EVs for each battle
Vitamins (Carbos, Protein) allow you to increase your Pokémon's EVs by 10, however if the EV count is above 100 on the stat they fail to work. So using 10 vitamins in the two stats you want to raise helps.
HP: HP Up
Attack: Protein
Defense: Iron
Special Attack: Calcium
Special Defense: Zinc
Speed: Carbos
Now the actual EV training. Here are the spots in Platinum:
HP: Bidoof (1 EV, Valley Windworks)
Attack: Machop (1 EV, top end of Route 210)
Defense: Geodude (1 EV, Iron Island)
Special Attack: Gastly (1 EV, Old Chateau)
Special Defense: Tentacool (1 EV, most watery areas by surfing)
Speed: Starly (1 EV, Route 101)
Hope I helped!