Marinate for 1-2 hours or overnight. In a pinch, mix and cook right away. You can also freeze the meat in the marinade at this point for use at a later time.
Place in slow cooker that is lined or sprayed well with cooking spray. Cook on high for 3 to 4 hours until pork is tender and will pull apart easily.
Remove from crock and shred pork, using two forks to pull meat apart.
Place tostadas on large baking sheet. Spread refried beans on each tostada, then top with layer of shredded pork (about 2-3 tablespoons), followed by a layer of shredded cheese.
Bake at 350 degrees for 5 to 10 minutes
To serve topped with chopped lettuce diced tomatoes salsa and sour cream