If u gonna use your hand, use fingers 3 and 4, the longest finger and the "ring finger" not the longest and the "point finger" (sry for not knowing a better way to name them

). The reason being, well u figure it out, try to do the spiderman motion and slap the lower part of your palm in the two differrent ways. Notice that the 3rd and 4th finger are the strongest ?
Anyways when you do it, you cant start "digging" cause your point and pinkie finger will make your hand stop. When you got it like this try and keep it air-thight so that you dont bring your hand in and out, you should instead be moving the fingers up and towards you (spiderman motion). make sure you just use the 2 fingers not the whole hand (we gonna save that part for later). start out slow and try and pick up speed as longer u go, will be a bit painful if you dont get it right the first couple of tries, but then just lower the tempo again and rest a bit while going slow
To make her cum externally, she cant be tenced at her lower stomach, this makes it almost an almost impossible task, wont go into detail here, just know that this is a fact for now. So make her relax and enjoy the moment totally , if shes not already, by gently rubbing that part of the belly (you will feel when the tence mucles start relaxing)
Now for the killer. When you start picking up speed a bit, assuming that you are doing it right, you will hear this sound, like water is moving around inside, then you move as fast as you can, tence your hand, and use the whole hand to make sort of the same move. Since your moving the fastest you can, your hand will move a bit sideways aswell. from the splashing sound to her screamin wet orgasm should be 10 sec aprox. If you get a hard time here, try to shock her system by sucking her clit just before you tence your hand. (Dont take your hand out while doing so, or even stop moving it!!)
This shit is fool proof. Read somewhere in a book that a study showed that only 20% of the girls cam cum externally.. Utter BS if u ask me ,, it never fails if you only know what you are doing.
Edit: Fun fact
There's this village in Southern part of Afrika where the ladies, in order to be allowed to be married, needs to cum like this. But not only cum, she needs to cum on a freakin plank thats 70 cm away! Every lady thats married helps out, and guess what,
They ALL make it!