With Legion pre-patch, the newly-added Wardrobe system will be implemented. If you want to get The Fabulous achievement, you need to collect 15 appearances for every slot including your shirt slot! This is your opportunity to earn some nice gold!
Most players only have like 2 or three shirts and if they want to complete the achievement asap, they will buy tons of shirts from the Auction House. This has an immense potential for Tailors! Why? Players are bored and sitting on piles of gold at the moment and since there will be nothing valuable to buy when the pre-patch hits live this is where you get your opportunity to grab some of their gold for yourself.
1. Get Tailoring. Shirts only require Wrath of the Lich King mats, so the required Skill level is just 425 or even less.
2. Get all the needed plans. Click here for all tradeable shirts crafted with Tailoring.
3. Gather materials for your shirts. All of them should be available really cheap on the Auction House should you have no time to farm them manually. Note some items may be expensive, depending on your realm.
4. Craft your shirts, TONS of them... not 10, 20... but at least 100. Put them in a separate guild bank or whatever. The more articles you're going to put on the auction house the more gold you're likely to earn.
Write in trade channels that you're either selling shirts for the achievement separately or as a bundle (25 shirts at once) which instantly completes the achievement.
Note that shirts are going BoE in Legion so you won't be able to trade them and they will bind once equipped. A pity is shirts aren't stackable, but that isn't particularly a problem. Go buy yourself a cheap guild or create a new Guild and put tons of shirts there to be stocked when Legion pre-patch hits live realms.