Add your flank steak to a container that has an airtight lid.
Add in your olive oil, white wine vinegar, kosher salt, black pepper, cumin, garlic, orange juice, lime juice, lemon juice, jalapeno, and cilantro.
Seal with your lid and shake so that the flank steak is coated. Refrigerate for 30 minutes to 1 hour.
Heat a large skillet over high heat. Remove the flank steak from the container (keeping the marinade) and sear on each side, about 45 seconds - 1 minute per side.
Add the flank steak and all of the marinade to your slow cooker. Cook on low for 4-6 hours. Cook until it's falling apart tender. If it isn't falling apart tender, it needs more time. Thicker cuts will require more time.
Once done, shred with a fork (it should be falling apart at this point), and serve in tacos or on rice!
Notes
The meat should be falling apart tender when it's done. If it isn't falling apart tender, let it go longer in the crockpot. Thicker cuts will require additional time.