The first few times you try the stunt, exhale very hard so the flame is extinguished faster. It is intimidating to have a flame in your mouth. Finesse will come with practice. If the flames reappear once you have removed the torch from your mouth, exhale with any remaining air in your lungs and close your mouth. This will extinguish the flame.
Many performers stick their tongues out to guide the torch into their mouth.
Some find this helpful, but be careful not to burn your tongue on any metal parts. The torch need only go in your mouth far enough so that you can engulf the wick. Initially, hold the torch for a few seconds then remove it. NOTE DO not inhale. Exhaling slightly will push the flame away from the lips and mouth.