I have only done it once, but I did it right! You need to measure the tube from the mouth to the stomach, and mark it. Depending on the age of the puppy, depends on the size of tube.
If the puppy begins to gag, you have the tube in the wrong pipe, and you could potentially kill the puppy by filling the lungs with fluid!!!! So please, there are many Youtube videos on this, I suggest you watch them over and over again. Also, if you have someone who has experience with this, like a vet, (my vet came to the house and helped supervise my first effort), I would suggest it. It is very scary in my opinion, but it is good to know how to do it if you get a fading puppy who will not eat, just in case!!!