I've read about two different ways that you can go about making this. Both include a can of sweetened condensed milk. The first way is to boil the can, unopened, and completely submerged in a pot of water for 2-3 hours. The 2nd was to submerge, (in water), in a crockpot for 8-10 hours on low.
I chose to try the latter.
Looking back, I think I should have heated it on "high" or let it cook longer, then I could have skipped the additional step. Next time I might try the stovetop method.
If you've got a can of this stuff in your pantry, give it a try!