Here's a basic list of what you'll need:
A Pokemon with Flame Body or Magma Armour (Talonflame is a great choice). This allows you to half the time it takes to hatch an egg, the Hatching O-Power is also helpful.
A valuable and breedable Pokemon (starters, rare Pokemon etc). Gen 1 starters seem to be the best choice here.
ORAS is a good addition to have, however it is not necessary (It helps with the Hoenn Dex)
To start you want to finish the Regional Kalos Dex (all 3 dexes) by seeing all the Pokemon. This can be done by simply catching all the Pokemon in the specific Pokedex, and raising them up to the level at which they evolve. Next, you must breed the fully evolved versions to create an offspring. Use your Magma Armour / Flame Body Pokemon now (put them in your party) now. If the line has 3 evolutions, breed 2 and raise them both up. Do each specific Pokedex at a time, starting with the Central , next doing the Coastal and finally finishing the Mountain Pokedex. Version exclusives can be obtained by simply catching your version exclusive and putting it on the GTS for the exclusive Pokemon in the game you don't own (Clauncher for Y, Skrelp for X etc). Certain Pokemon such as Munchlax can only be obtained by breeding, so keep that in mind. Catching all of the Kalos Pokedex has two good advantages:
Place the Pokemon in their respective place in the box and repeat.
Pokemon you can't find in XY
To get these Pokemon, simply do the following.
Breed your starter (Gen 1 is preferred) with a Ditto to make offspring.
Put these offspring up and ask for the Pokemon.
Do the process with the Pokemon you have obtained.
Hope I helped!