You actually do all the work yourself, just study FAA Order 8900.10 and it outlines the process for you. However, it can quite frustrating. Presently according to my sources, the FAA is issuing NO new operating certificates. They claim they are over worked and under staffed to do any more than deal with the operators they have. Personally if they would stop trying to invent regulations to enforce and just enforce the regs on the books they would have many of the problems they presently do. A 125 certificate is considered less important than 121 or 135 certificates so those applicates would be handled last. I did a single pilot certificate several years ago and it took my over 6 months, so I would plan on a year or more. A quicker route would be find a 125 certifcate for sale. However, legal you can not buy just the certificate, you have to buy the company holding the certifcate.