How do you become a professional Dog trainer?

My aunt went to college to become a trainer- this was like 20 years ago so I dont know which one. I would try goodsearch and see if any 'Dog Training Colleges' show up I wouldnt trust any that allow you to "work from home" since its much better to have personal experience.

I think one of your best bets would be to sign up at your local shelter for dog walking,just to get some experience with working with different breeds and types.Most shelters hold different types of classes,whether it be puppy training/Dog Obedience/Clicker Training,etc.
I know that our shelter actually gets some of the shelter dogs involved in those classes,and as a Volunteer,you could probably help and participate with them.
I myself want to get into the clicker training. lol
Once you get your feet in the door,you'll be surprised at how much you can learn,plus it's a good way to decide how serious you are with that, as an actual career.There are many,many doors that can open for you with a plethora of information,on steps that you can take to go further with this.And the best part of it all is that volunteering at the shelter is Win/Win situation,not only for you,but for the animals also.

if you go to your local shelter and you ask them for a registration form and fees they might hold classes there for you

I know when I worked at PetCo, the store would offer lessons you could take. I can't remember if the store paid for them or not, but it was a series of classes over the span of weeks or months, and at the end you are tested and could become certified. Perhaps check around local dog trainers and ask how they became certified :)