The fastest method for shiny hunting Ralts is to Dex Nav search for it or to Masuda Breed. (Personally I would go for Dex Nav because it took me 300 something encounters to find mine without charm.)
If you already have it, the Shiny Charm ups the odds of finding a shiny. Assuming you want the shiny as soon as possible, I wouldn't recommend going for it if you don't have it.
For this, you will need a Flame Body Pokemon, a member of the Gardevoir line (Female, if breeding with Ditto gender doesn't matter) to make sure you get the right offspring , and a foreign language Pokemon that can breed with the Gardevoir line. This includes the Gardevoir line but foreign Ditto is usually best. (Examples are Gardevoir and foreign Ditto, foreign Gardevoir and Gallade). You can attach an everstone to a parent to pass down the nature but it has to be the parent with the offspring. While the shiny chances are boosted, this method is time consuming if your luck is bad. (Accurate Masuda guide)
For this, all you need is to have already caught a Ralts and Repels (recommended but not necessary) I find that the best way to do this is have a Pokemon with moves that have lots of PP. You can use Synchronize to try to get a specific nature but it won't guarantee that nature. Go to route 102 and search for Ralts using the PokeNav (you might have to run around a bit if it isn't showing up) and sneak to it. Once you encounter it, beat it or catch it (you can also run away, run around for a bit, them search again), then search for it again. This might not take as long as the Masuda method but the shiny could take a while to show up. Do be warned that if you find one with the Egg move Memento or if it's level is 9 or higher and has Teleport, you could fail. Do note if you go too fast while sneaking, it will scare off the Pokemon and you have to search for it again. (Accurate Dex Nav guide)
Good luck and have a good hunt :)
Source: Experience shiny hunting in Omega Ruby with both methods (I have a shiny Gardevoir using Dex Nav) plus I watched lots of videos