When you reset the game you always start at frame 0. Assuming you're resetting and getting to le legendary as fast as humanly possible, you'll be hitting a frame within a small cluster which may or may not be shiny.

For normal wild Pokémon, just hunt normally over a long-ish session.

For static Pokémon, every 50-100 soft resets go to the battle factory, play a battle video if you have one, do a battle and lose it, record it. Then after each reset in front of the Pokémon play and immediately close that video as it will move you forwards (or sometimes backwards) to the frame you were on when the battle started. Get a new video every 100 frames or so.

After you get your shiny, you'll want to get a new battle video if you want to keep shiny hunting. Or you can keep using that same video to get more static shinies with the same natures and IVs, but at that point you'll just be doing RNG manipulation.