If you only want one type of specialist, first get one specialist. Switch the specialist to whatever you like (if it isn't already that one), lets use Scientist as an example. Click on the Scientist category button, not the plus or minus buttons. It should become highlighted with a yellow border. Turn the specialist governor ON. Until you click the highlighted Scientist to turn off the preference, the governor should only assign Scientists, as long as more are allowed. If you turn off the preference, don't forget to turn off the governor, or you will get a lot of reassignments!
For the most part, I just use micromanagement with the governor off but I like to micro. The above is how to use the governor.