You go round all of sinnoh and see every pokemon. It is easy to complete but will take a long time. You will not be able top catch them all without trading but you can complete it
without trading
There is only one legitimate way to complete the Sinnoh Pokedex. To complete the Sinnoh Pokedex you must trade for the missing Pokemon with Pokemon Diamond, Pearl or Platinum.
Just go to Pokemon platinum central and click on pokedex and it shows you where to battle all the Pokemon in sinnoh.
First complete the sinnoh pokedex, 210 Pokemon
complete the sinnoh pokedex and talk to professor rowan.
After you go to the Pokemon lab for the first time, Professer Rowan will ask if you would complete the Sinnoh pokedex. Answer "Yes". He will give you the Sinnoh pokedex. If I didn't answer your question right, try Psypoke.com If you don't have Pokemon Pearl/Diamond/Platinum you can't get the Sinnoh pokedex.
You can either hatch them or run into them with the pokerader. To get the pokerader, you have to complete the sinnoh pokedex.
Walk every square inch of Sinnoh and battle every trainer
I am not going to list the list here, but here is how to open it online:1. In Google, type 'national pokedex platinum'.2. Press 'return' or 'enter'.3. Click or Double click on 'Pokemon Platinum- The National Pokedex'.Then the Sinnoh Pokedex will appear (even though it says National Pokedex, it's the Sinnoh Pokedex with the location and/or a trainer with the Pokemon)!
A way to complete your sinnoh dex is to battle every trainer, Nintendo made it so if you battle every trainer you will complete your sinnoh dex.
in the national pokedex, it is chikorita, and in the sinnoh pokedex, it is rotom
in sinnoh 210
in the sinnoh pokedex dusclops, in national pokedex aipom