1. Obtain a member of the Charmander line and Ditto from two different regions.
2. Complete the National Dex for the Shiny Charm.
3. Obtain a Pokémon with Flame Body.
4. Obtain Hatching O-Power level 3.
5. Head to the Battle Resort's Day Care.
6. Breed like no tomorrow. Running on all cylinders should let you hatch 5x eggs every 3 minutes.
Alternate method:
Ask someone for a spare shiny Charmander.
I haven't tried for Charmander yet, but I've hatched shiny versions of Chikorita, Totodile, Torchic, Froakie, Eevee, Ralts, Skorupi, Durant, amongst others I can't remember.