You have to battle them.
every pokemon from that game and generations before
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
yu have to get every Pokemon on the sinoh dex
You can get Pokemon from the older versions on Pokemon Platinum, but I'm not sure if it includes every Pokemon from every game. You'll need to finish your Pokedex, then get the National Dex.
this question is worded weird but if your question is "where do you get all thePokemon in Pokemon platinum?" then the answer is to battle every trainer in Sinnoh.
first you have to battle with every pokemon like i did! it took me a long time but i did it on platinum i also have all legendary pokemon
You have to see every pokemon in the regional pokedex. Then you have to obtain the national pokedex from i think professor oak or professor rowan.
You don't have to get any Pokemon to get the national pokedex in platinum, but you do have to see every Pokemon in sinnoh. Don't worry, if you're thinking "Oh no! How the heck am I gonna see all those Pokemon?!", you shouldn't. If you battled as much as you could, you saw every Pokemon in sinnoh, even if you weren't a fan of tall grass.
You have to complete your pokedex the oak gives you a call
yes you hold down r and l and every Pokemon will apear in your pokedex i do not know the code though it don't work
Yes. Every single Pokemon from every Pokemon Game is part of the National Pokedex
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.