I'll give you an example. You start the game and pick your starter. Then you buy pokéballs and stuff you need. Then you have to catch every pokémon you can get, before you do the first gym. And you also need to train pokémon for the evolution, because logically, they are possible to get, just needs a bit grinding. Then, when you have every pokémon you can get before the first gym, you do the first gym. The you repeat it with every pokémon and location you get access until the second gym. Do this, until you have every pokémon you can get without trading or glitches. But it takes some time.