Marinate your cubed steak for at least 30 minutes. While your steak is marinating, cut your peppers and onion into large cubes. Also, use this time to soak your kebab sticks so they don't burn on the grill.
Skewer your meat and veggies alternately. Set up your grill for direct cooking.
Cook on each side for 5 - 8 minutes at 350 - 400 degrees on your grill.