My girl liked only one finger until i got a magic wand. I use the magic wand on her till she cums the first time then i start in slowly using j lube and 1 finger at a time, Use one finger for about a minute or 2 before moving to 2 fingers stroking her g spot with the cum here motion, Making sure ur putting enough pressure on the outside with the magic wand on her clit.
At this point let her cum again, She'll be shaking around & convulsing all over the place by now, Here is the point where u can move upto 3 fingers, Remember u should never be forcing ur way in, at this point her pussy should almost be wanting u in there, She should still be coming around here or just about to again & when she does cum move to 4 fingers leaving the thumb out, Make sure ur hand faces north / south rather than east / west the vagina will accomidate u better.
Around the time she cums next if she still has enough energy to keep going u will be able to get the thumb in, Her head will be in lala land so she will have no idea how deep your in, This is where listening to your partner helps cause ur not going to get the fist in the first time maybe not even till the 5th time u do this but it will eventually it will go in past the knuckles & ull be able to get your fist inside clenched.
Don't move it around too much straight away let her get used to u been in her like that, After has settled a little u can start slowly moving in & out but not so much that u pull all the way out, Just small round movements, U will notice as she cums acustomed to your fist being in there she will start to either pull ur hand or start grinding back against you fist...
Time & Practice is all it takes, Hell i can get a fist in my ass so why not a vagina??
Tip - I always have some type of porn on in the background, I find the sounds of others humping & grinding stimulates my senses & turns me & the fistee on a little more help the person getting fisted to relax & go with the flow more, rather than being tense.