Well let's see here...
First of all, don't do that. It's retarded. Second of all just get the certification. Even if you get some shitty internet certification (Google ISSA, should be simple enough) it's better than having no certification. Lastly, once you get certified get yourself some insurance. (Just google personal trainer insurance or something) It's about 100-150 bucks a year and covers you for something like 1,000,000.00. If you hurt someone without any formal training, and without any insurance, you are so fucked beyond belief.
Once you do step 2 and 3, then you can put up flyers. Though, you need to make sure your campus gym is ok with that. They may already have some trainers there, and would not allow you to come in and train without having a real job with them.
Hope that helps.