The person who needs an EV trainer transfers the PKMN to the EV trainer and specifies what EVs he wants for each PKMN and how he/she wants them to be EV trained (no AR items that are illegit, hacking, etc. can be done on the PKMN), and the EV trainer follows the instructions precisely. In exchange, the reward is usually a clone of the PKMN, a credit in the person's thread, items, etc. Sometimes people ask for references from an EV trainer to prove that the EV trainer is reliable.