How do you put a buttoned shirt under a leather jacket in GTA 5 Online?
Since the directions on SevenSins are a bit confusing (at least to me, and probably due to a language barrier), I'm going to rewrite them specifically for MC outfits.
Bear in mind this is simply a rewrite for clarification; I take no credit for finding this.
1. Start a motorcycle club.
2. Bring up the Interaction menu, then go to the Motorcycle Club President menu, go to Manage MC, go to MC Style.
3. For Style, go to Rocker.
4. Pick the outfit style you want. Outfits 2, 4, 6, and 8 have the Black Classic Biker or Worn Black Classic Biker styles with shirts underneath. Pick whichever outfit has the jacket/shirt combination you want.
5. Start a survival mission.
6. Once the mission has started, bring up the Interaction menu and go to Style.
7. Go to Outfit, select any outfit, and confirm your selection. Your outfit will not actually change. Do this several times with different outfits.
8. Do the same thing as #7, bit with the Bike Helmet entry in the same menu. Again, your helmet will not actually change. Do this several times as well.
9. Quit the mission.
10. When you get back in free roam, repeat step #2.
11. For Style, select None.
If successful, when you pick None in the last step, the outfit you're wearing should not change. It should remain the same MC outfit you selected earlier. Now you can change other elements of the outfit, such as pants or boots, as you see fit. Then save the outfit.