Well, take it one step at a time.
Start off by getting every Pokémon available in Kalos. Complete the Central Kalos Dex, and then do the Coastal and Mountain Dexes. You may need to GTS for Pokémon exclusive to the other game.
Then look at the rest of the Pokédex. Just go through and see which Pokémon you have and which ones you don't. If you have other games, look at your wanted Pokémon's locations (I used Caterpie as an example) and see if any of them match up with other games you have. If you find some like this, catch them and then Pokébank over to X/Y. If you don't have any other games/still have some Pokémon left to catch, just GTS.
The biggest problem you'll run into is that Legendary Pokémon, especially event legendaries, are very hard to obtain. In order to get those, you may want to edit your profile to say "Want Legendaries for Dex" or something to that extent. Then offer to trade, and have some Pokémon ready with nicknames like "Mew plz" or "Shaymin?"
Another good strategy for getting the legendaries is to find a friend who hacks/clones Pokémon and get one from them. After you get one high-value legendary, you can clone it to get the others pretty easily.
Good luck filling that Dex!
Source: Experience
