With all due respect to what has been previously posted and depending on what your goals are:
Lighting a mannequin head is no more useful than observing how light falls on a face in real life or how they light in movies. Where it could be useful is in exposure. You should be confident of your exposures before you meter, if you need to meter at all.
For every mannequin head you shoot you could learn a lot more by using a live model. Any and every model you can get your hands on.
Here's my suggestion for lessons in portraiture: Write a script for shooting. Note down exactly what you'll be saying and pre-visualize your timing around the statements. Write a thirty second script for about fifteen shots, take a break, observe what you achieved and fix the lighting if necessary. Then launch into the script again or deeper to achieve a wide variety of expressions. Shoot for another thirty seconds and you should be done. In the first event, don't touch your lighting at all, you should have it fairly down and have an intuitive estimate of exposure. Talk, observe, place your model and go immediately into shooting. The model will do anything you want for about the first thirty seconds. Adjust for the second event if you want to.
Variety of expression and connection with the model are initially more important than the lighting. You will also have accomplished a concrete task instead of just speculation. Write different scripts for different people and also observe the model when they first show up for any gestures, expressions you can add into your script. Always believe it's going swimmingly. Have faith in your capabilities and make sure that your model has trust in your skills.