I seem to be having problems trying to RNG egg's, to be specific a shiny litwick. This is honestly my first trying to RNG and I can't really find out where my problem is. I start the game and use sweet scent to immediately catch a pokemon and use pokecheck to get it's exact IV's. Then using all the information I found a good frame to hit and I tried a couple of times but I never got it and eventually I got a litwick with a nature that was nowhere in the list of possibilities. I just don't quite know what I'm doing wrong. Does saving after you catch the pokemon as you log in mess up the seed? I also know that with all the moving NPC's near the old man can change the frame but are there any tricks or advice that you guys can give me to counter that problem and also and tips to figure out what I'm doing wrong.
When you hit the wrong seed, try using the Adjacent Seed Tool and compare the IVs to see if you're hitting the wrong Timer0 or second.
Saving after you catch the Pokémon doesn't change anything, everything about it is set by that point.
There's nothing you can do about NPCs except deal with it; however, the top floor of Celestial Tower is NPC-free so you can RNG Litwick there without dealing with them (just make sure to use the right floor's encounter slots).