In a small bowl, combine the chili powder, cumin, garlic powder, onion powder, salt, and pepper. Stir it up.
Generously coat both sides of the flank steak with the spices and place in the bottom of the slow cooker.
Top the flank steak with the peppers, jalapeños, onion, and garlic.
(You may worry about there not being any liquid in the slow cooker, but have no fear. The steak and veggies will give off enough liquid.)
Place the lid on the slow cooker and cook on low for 8-10 hours. You *can* cook it on high for 3-6 hours, but I would recommend the low heat over the high heat for this. Cook until the steak is super tender.
Remove the steak from the slow cooker and shred. Personal preference: I like to cut the steak into a few sections (against the grain) so that the shredded meat isn’t super long.
Place the shredded meat back into the slow cooker and gently mix everything together.
You could do absolutely anything with the barbacoa. But make tacos first. Heat up some corn tortillas, fill with some of the meat and veggies, top with avocado (or guacamole), salsa, and some cilantro. Enjoy!!