There is no fastest way really, you can luck out in any method but in theory the fastest way is probably chaining.
The explenation on it is long but the gist of it is that you go into tall grass and the Pokemon you find will be the one you're trying to get a shiny off. You have to defeat or capture it or it will break your chain. Then you use the pokeradar and go to the most shaky grass patch and repeat the process.
Here is a youtube video explaining it more in-depth.
You can read up on all the methods here.
Hope I helped!