They will hold a legal qualification of some kind. Currently to be a Police Station representative  - "Police station rep" they will usually have to be " accreditted".  You will  either hold a qualification in Criminal Law i.e. a Law degree or will need to pass a exam in Criminal Law. You will then have to submit a portfolio of police station attendances demonstrating your knowledge and practice before undertaking a Practical Test

As a police station representative, you may, if you are self employed work long hours. Police interviews can onften be at any time of the day or night. Police forces interview detainees all year round. Police station custody suites are open 24 hours a day, 7 days a week and 365 days a year inc public holidays and Xmas. As a Police station rep your advice at the police station could make the difference between the police taking no further action againgst your client or a lengthy prison sentence. 

You have to be courageous and assertive to be a police station rep. The police will view you as an obstacle to convicting your client. The client may be obnoxious, aggressive or angry or downright rude. You may be working in a highly charged atmosphere where a cool calm head will be necessary. You must be able to  bring reason to the unreasonable and be able to consider other points of view.