From my own experience, the process itself is not so complex, if your employer is legally registered and have proper documents, and is actually willing to give you work permit. The prerequisite is to have a business visa, that you have to get from outside of Thailand, the company will have to provide you the necessary documents. You cant check for the list available online. Then converting it to the right visa along with work permit is a 1day appearance at the designated center. Mine was done by an agency, handled by HR dept, i just had to take a simple medical test and then appeared in person to that center in Chamchuri square, waited for an hour while the agent did all the running, called infront of an immigration officer sitting there. She asked me about my work an basic socializing questions, and issued the 1 year reentry permit and work-permit then and there. I think it depends on the company, if everything is in order and they are actually willing to give you work permit, rest should not be much trouble. Good luck!