You are correct. Soft reset until it's shiny. I've done it and sometimes you're lucky and can get your target in a short time (I got a shiny Ho-Oh after only 47 soft resets on HeartGold), but other times it can take forever (27k+ for a shiny Virizion).
But be aware that a few legends are shiny locked,such as Reshiram and Zekrom in gen 5, and all legendaries in X and Y. Most of them aren't locked in ORAS (only The weather trio and Deoxys), so you can have a pretty easy time getting a lot of shiny Legendaries with the lowered odds.
Hope I helped!