So you most commonly want perfect IVs (31), and to do that, you need to start off with Pokemon that have perfect IVs. In X&Y, there are 2 ways to guarantee catching a Pokemon with some perfect IVs:
- friend safari (2 random perfect IVs each)
- radar chaining (1 random perfect IV at chain 30-39, 2 at chain 40-49, 3 at chain 50+)
Get a bunch of different ones, obviously because different Pokemon caught under these circumstances have different random perfect IVs. Then go to Kiloude City, and ask the Ace Trainer in the Pokemon centre to judge your caught Pokemon. He will tell you what stats they have perfect IVs in (when he says "it can't be beat!" after naming the stat). Mark the perfect IVs with the marking system on the summary page of the Pokemon. After that, you need to use the Destiny Knot, which will guarantee 5 random IVs passed down from both parents. Just get either parent to hold the Knot as you put them into the daycare.
You can also use the Power items to pass down specific IVs, but it's still unclear to me whether that stacks with the Knot. However, you probably set the nature instead, which will require an Everstone on the parent with the nature you want.
After all that, you need to get on your bike and get hatching, because you will need to hatch dozens of eggs before you manage to hatch the Pokemon with the IVs you want. You can also replace one of the parents with the offspring (ew, yes, but Pokemon has no concept of incest) which have better IVs and are therefore better for breeding. The more the IVs overlap, the more likely the offspring are to inherit them.