Simple. Stand far enough of each other to be out of range for limbs and held tools. Now walk naturally and glance on the point where you are comfortable to hit and keep walking out of that range in the same pace. This will teach you placement and distance well and later on add the knife and go for the unseen stab or other hit. A pro does not show his knife not in the hand and not later on. See the big picture.
Another drill; Lie on the ground and have a partner stand close and try to step into you. The point is to stomp into the body (incave a lung for example) and your job is to move at the last minute out of the way but to stay close to the legs so you can work and stop the other from kicking. Change places and see how this works for you. The one standing needs to to maintain his composure and not overstep. Simply walk and breathe on the person (if he fails to get out of the way) and don’t secrifice your posture of composure to gain any of his. It is a road leading downstairs.
lower yourself to one knee next to your friend on the ground. Take a sharp branch or a knife or whatever you choose and start to slowly push the tip into the body and face of your firend. Make it real so he learns otherwise you are doing both a disservice. The one on the gorund has to feel which part of the body has to move and where. Imagine the tip deviding the body into several parts. It is a pivot point and decide by using your senses and mind which part is best pivoted. For example a stab to the shoulder blade will not be helped by a roll to the right or left if the tip is on the inside of the bone. you can however roll forward and glide the tip on the body so you are maybe cut but not punctured. See what works for you.