Load round balls, boys - in 12 bore - Bluedot, Herco, Unique, Hodgdon's Longshot or AA's Steel are the powders.
Ed Huble's thread here that's been running for years 12 Bore From Hell and data in it that can or will help you get started.

Think about a wad or wad 'arts' that can help keep an undersized ball centered in the bore - worked & works for me.
Too- Ed and others now having possession of his 12 bore wads that hold .58 calibre minnie or other bullets, allows use of bullets of 500 to 600gr. in 12 bores, giving the power of an old .577 BPE. Interesting stuff indeed.

The reason I suggest round balls is they work well in smoothbores as well as rifles.