This achievement is pretty self-explanatory. Start the game, and play through the opening cutscene. You will not be able to load your save from Dragon Age: Origins until after you have passed the first battle. Once you pass this battle, you will be given the option to modify your appearance, change your portrait, and either select a preset group of events from the first storyline, or load your own save.
Just for fun, here are the three groups of events:
The Hero of Ferelden: Human noble, good guy. Took Morrigan's way out, Alistair is king.
The Martyr: Dalish Elf, female, described as cynical but out for the greater good. Alistair & Anora rule, hero dies.
No Compromise: Dwarven noble, described as ruthless. Alistair exiled, Loghain kills the Archdemon, Anora rules.
Import your own save. When you select this option you are allowed to choose a save of one game from any of the DLC, or your main game playthrough. Make sure you choose correctly if you want Awakening & Witch Hunt (if you played them) to show up.
NOTE: It is possible to get the achievement without a completed save from DA:O. You only need to complete an origin story.
***UPDATE!!! 31 May 2011*** (Confirmed by Gryphonskull)
Lushion Rudd says: "The patch came out today, and it did fix the epic achievement. I loaded up my last checkpoint that begins the battle with Orsino, that then moves onto the final battle with Meredith. Upon beating Meredith and skipping the cut scenes I got the achievement. Loading the post game check point did not work."
So just load up your checkpoint just before the final battle, and you should get it.
***UPDATE! 10 May 2011***
Thanks to CowsMakeNoise, we have some concrete answers. At this time it is POSSIBLE, but unlikely to get the achievement. Each playthrough after your first has a 21.4% chance of getting you the achievement. More & official word from bioware found here: http://social.bioware.com/forum/1/topic/302/index/6597490/3#...