First off, what's a dragon priest.

And secondly, how do I do this!

A rare creature that can use dragon shouts. High Conjuration level.

I'm pretty sure you can't conjure a dragon priest unless you get the tome as a quest reward, in which case, I have no idea and would love to know.

if you do the dragon priest mask quest (its not actually a quest..u find the dragon priest masks on dead dragon priest bosses) you will eventually unlock the final mask which has a low chance of summoning a dragon priest when your low on health.