1. Study the current metagame. Sources like Smogon or news sites that cover VGC rules and formats are helpful. Understand all the varying strategies and their deviations. Rates of success versus different team combinations and why, synergies between different team members, etc.
2. Obtain the Pokémon in question. Take into account: IV spread, nature, ability, egg moves (if any), EV spread. Even if you're starting from scratch, you can easily get the nature you want by utilizing a Pokémon with your wanted nature with the ability Synchronize (such as Abra line, Ralts line, Espeon, Umbreon) and catch a Ditto. 50% chance it'll also have the same nature. Then use an Everstone and pass that onto the child Pokémon before breeding for IVs. If you don't have the right Ditto nor the right Synchronize Pokémon to start, either trade for one or catch one (in ORAS, Dex Nav-ing a Synchronize Ralts shouldn't take too long; it'll come down to randomly finding one with the right nature before you hunt down a Ditto to make everything easier in the future). For example, I personally have one Ditto of every nature, which makes it incredibly easy for me to have any breedable Pokémon of any nature I want.
3. Breed the Pokémon. Look up guides to breeding competitive Pokémon. In all likelihood, to speed up the process, you'll need: a 6-IV Ditto, Destiny Knot (on the Ditto to pass IVs), Everstone (on the other parent to pass nature), Flame Body Pokémon, Oval Charm, max hatching O-Power. You'll likely have to breed across more than one generation in order to fill out the IVs.
4. Once you've bred a Pokémon with the right IV-spread—for example, 31 in Attack or Special Attack for sweepers, with 31s in HP, Defense, Special Defense and Speed; check an IV judge in your game (Battle Resort Pokémon Center in ORAS) to confirm—you'll need a EV-boosting item such as Power Anklet for Speed, Power Bracer for Attack, etc. and Pokérus to speed up EV training. You're better off trading for a Pokérus-infected Pokémon as you're very unlikely to randomly encounter it in your normal playthrough.
5. With the appropriate EV-boosting item and Pokérus on your competitive Pokémon, bring a Sweet Scent AOE Pokémon with you for horde training. Personally I use a Venusaur with Earthquake/Petal Blizzard. Example, if you're EV-training for Speed, attach a Power Anklet on your Pokémon, turn on Exp. Share, head out to Route 104 (west of Petalburg City) and spam Sweet Scent in the grass to fight Zigzagoon or Wingull hordes. Each battle should yield 50 EV points to Speed if you're properly set up (1+4 with Power Anklet * 2 from Pokérus * 5 enemy Pokémon = 50 EVs).
6. If you're able to obtain multiple QR codes for Blissey Secret Bases, you can bulk grind EXP and reach level 100 on any Pokémon in roughly half an hour.
7. Repeat this five more times to fill out your party.
8. Obtain the items you'll use on each Pokémon, such as berries, Choice items (Scarf, Band, etc.), Focus Sash, or anything else that synergizes with your strategy. This will likely involve grinding the Battle facility for Battle Points if you're starting from scratch.
...The most important part is that even if you have a fully set up team, you're not going to get anywhere unless you understand strategy. No competitive Pokémon match is a straight empty-minded offensive onslaught that gets you through the main game. In competitive battles, I'd argue it's 90% mental games involving predictions, baiting, setting up, and numerous counters based on what the trends are at the time.
