1) informal, meaning you just entrol into any university and just take courses, you plan your schedule, you do everything (this is what I did)
2) apply and get accepted into special post-bacc programs designed for pre-med / pre-dental students. My underestanding this, these formal pose-bacc courses start you from scratch (100 level biologies, 100 level general chemistries, your 2 orgo courses and all thier labs)
Check the post bacc forum on here, there are tons of information. Although, most of the posts are for pre-med, you'll find quite a few for dental.
By the way, not all post bacc are necessarily lower division classes. Some are academic enhancers with upper division science classes.