You have a body and a head...now what?
I will show you, step by step, the easiest and safest way to install a doll head onto a body. Follow these same steps to remove the head as well.

You will need:
Doll body
Doll head
Microwaveable heating pad (electric heating pad can also be used)

You will need to soften the vinyl head in order to easily pop onto the neck mechanism of the body.
​Heat your microwaveable heating pad in the microwave for 2-3 minutes, depending on the size of your heating pad and the power of your microwave. 

Place doll head on heating pad once it is heated and removed from microwave. 

Wrap the doll head up in the heating pad.
​Leave doll head wrapped up for 2-3 minutes.

Make sure the neck post of the body is in good working condition with no breakage. 
It should move about with ease back and forth, side to side. 

Remove doll head from heating pad after allotted time. The head will be warm and flexible/squishy. You will need to squeeze the head in order to pop it on the body. Gently squeeze and twist and press all at the same time, inserting the body neck into the neck hole at an angle. Keep squeezing, twisting until head is popped on.

Images of the squeezing, twisting and pressing.

Voila!
You now have a complete doll.

Use these same steps to remove heads as well.

Good luck :)