Quarter the onions and finely chop.
Prep the tomatoes.
Mince the cilantro.
Slice the cheeses into thin slices.
Cut the tortillas into strips, but not too wide.
Cook chicken with a little water, pinch of salt, the quartered onion and 2 cloves of garlic. Cover and slowly cook over medium heat. You are basically steaming the meat.
Cook until the chicken is cooked through and tender. Put chicken on a plate and let it cool for a while until you can shred it in very small pieces with your hands or a fork.
Throw out the garlic and onion. They were only used to add additional flavor.