The act of doing this is legally controversial and the laws regarding it vary by place and case. and some would say unethical. But for the pursuit of knowledge, what you are referring to is known as milking. It is extremely dangerous and I've done it on prairie rattlers here only twice out of the 17 caught. I HIGHLY recommend watching videos on snake milking because if the snake were to bite you it has the potential to kill you. but all you need is a jar with a cover made out of rubber/latex like hospital gloves, or thin leather like in cheap leather furniture. Stretch it over the hole and hold the snake at the base of its skull with a finger on top. Without harming or forcing the snake, make it bite into the latex and be sure to hold its head very very firmly but again, don't harm the snake doing so. You want the snake to not try to free itself to bite you cuz it will definitely try. Gently press the top of its head massaging the venom glands making the snake inject as much as needed. Then slowly and carefully unwedge its jaw without getting close enough to get poked. Or simply do this on the ground and once you have the venom quickly let go with its teeth still sticking in the jar, keeping snake at safe distance with snake staff or stick or whatever. Then using a very fine sandpaper, slightly roughen the sides of the dart tip. Dip in venom, and there we go! Note; venom will not work the same once dry. Poison your tips in the field not beforehand. A simple ***** on the finger can hospitalize you.