Learning to punch
Before you start to punch in training you should learn to push. Start by finding the areas on the body where your fist fits. If when you apply pressure to the body you hand wants to rock off it you need to reposition the hand.
Next get the alignment of the fist, wrist and forearm correct. The back of the fist, the wrist and forearm should all be aligned. If they are not you will damage your wrist.
Put the fist on your partners body and play with the angles of the arm. At the right angles you will need significantly less pressure to move your partner. Also it is important to find the parts on your partner where your fist fits. Without this your punches will run the risk of glancing off or your wrist being injured.
Start to push you partner using you fist on their body. Do not lean into the push and keep the shoulders relaxed. Experiment with this and develop an understanding of what different pushes do to your partners form.
Make sure that you breath out with the pushing movements. The sharper the push the sharper the breath should be.
Now try with striking. Start hitting into the skin layer of your partner. As they learn to develop their ability to dissipate the the strikes increase their depth. Get feedback from your training partner as to what is happening in their bodies. You should be able to know exactly what a punch will do before you hit your partner.
If teaching it is better to receive more strikes than you give. The is to stop you becoming too proud. Also that way you will not become a lazy instructor.