Basically, what you are doing is not adding new Pokemon, you are trchnically just editing an existing Pokemon so it resembles a newer generation one.
You first have to replace an existing Pokemon's sprite with the one you want to add (ex. Infernape) with programs such as Nameless Sprite Editor and then editing it's stats with YAPE or related programs so it resembles the stats of the Pokemon you want to add (Infernape). There should be many tutorials on how to do this. Just look through the Tutorials section and you will find some.