To do this, it's really not that easy, but I'll put it in simple (I think) steps.
Catch all of the Pokemon you can in ORAS (Whichever copy you have.) This includes all Pokemon at mirage spots, gift Pokemon, legendaries, etc.
You need a copy of X/Y. (Or both) This will help you catch all (I think all, right?) of the other Pokemon, leaving some out such as version exclusives.
You need to get the Pokemon you listed below:
The Kalos Starter you didn't get in X/Y
The Kanto Starter you didn't get in X/Y
The Johto Starters you didn't get in ORAS
The Hoenn Starters you didn't get ORAS
The Sinnoh Starters you didn't get in ORAS
The Unova Starters you didn't get in ORAS
The Hoenn fossil Pokemon you didn't choose in ORAS
The Kalos fossil Pokemon you didn't get in X/Y
And these ones below depending on which game you don't have (You need the ones from the game you don't have.):
(Pokemon for OR on the left, AS on the right)
(X on left, Y on right)
I've forgotten two short but important steps that you need to do.
Evolve all the Pokemon to their last evolution if you haven't already done so.
Take the older Pokemon (By older I mean a higher evolution than the first form.) you have, and if you do not that e previous evolution of it, breed it and get the evolution in the Pokedex, you can release it afterwards.
I know you might not have both X/Y or another and both OR and AS but it's the only way to do it absolutely by yourself.
You could also make a list of Pokemon you need, go onto chat, and ask if people are willing to help you get them. (That's what I did and I ended up filling up the Pokedex, event legends and all. (However I did use some GTS.))
Hope I helped!
(ANOTHER NOTE: As far as I know, between X, Y, OR, and AS, all Pokemon are available (Excluding event legends.) however I may be incorrect)