First, you have to have citizen automation turned on. (In the city screen, just below the buttons to whip- and gold-rush, are the production automation and citizen automation buttons. Make sure the right one, citizen automation, has a gold border.)
Next, you can click + or - next to a specialist type. DON'T CLICK IT TOO MANY TIMES OR YOU'LL TURN CITIZEN AUTOMATION OFF. The game assumes if you click it too many times that you *really* meant what you just did, and it turns automation off to try and do what you wanted.
Bottom line you want the citizen automation to have a yellow box, and you also want a gold box around the first specialist type that you want to force.
Once you do this, the game will add ONLY that specialist type until it is full and it has citizens working ALL tiles in the city. Only then will it try to add another specialist type.
Once you get the hang of it, you can experiment and force 2 types, etc.