Soak bamboo skewers in a plate with water for 15 minutes.
Preheat broiler to HIGH. Line a baking sheet with foil.
In a large bowl, combine steak cubes, bell peppers, and onions.
Add cumin, chili powder, paprika, cayenne pepper, salt, pepper, and oil to the steak mixture; mix until well incorporated.
Remove skewers from water and thread with the prepared vegetables and steak.
Transfer skewers to the prepared baking sheet and roast in the oven for 5 minutes; flip over and continue to cook for 4 more minutes, or until the internal temperature of the steak reaches 135˚F to 140˚F.
Remove from oven, garnish with cilantro, and serve with guacamole if desired.