If you are in Chiang Mai, go to the Immigration office near the airport, take your passpoort and all its inclusions and go to the drive through window (car, bike or walk) no more than 5 minutes.
Aneska J V Rensburg I do not believe that a police clearance is required. All depends on which immifgration office you apply at. Best go to your office prior and ask for their list of requirements.