First you need a C-Gear seed to hit. I'd recommend using Kaphotics' list of Desmume C-Gear seeds (page 24 of the RNG Research thread). Then you hit the C-Gear seed. Next you turn off the C-Gear. Make a save state and catch a Pokémon to confirm that you've hit it. Once you've hit your C-Gear seed, load your savestate and advance the MTIVRNG as many times as you need to in order to get your desired IVs, then you use the seed check codes available in PIDRNG to get Seed1 and Seed2 and input them into the RNG Reporter. Search for one that's shiny and either has the nature you desire, or has a synchronizer that works. And then you get to do the painstaking process of advancing the PID RNG 4 frames before that, make a savestate, and see if it turns out shiny. If not, see what PID you really hit, load your savestate, and advance the frame accordingly. That's really all there is to it. You need an emulator so you can make savestates and take a break during frame advancement.