The 'ar sound' /ɑr/ is an r-controlled vowel. Technically this sound is two distinct sounds (vowel sound+'r sound' /r/). It is presented with the name 'ar sound' /ɑr/ to distinguish the fact that the vowel portion of the sound is different from the various pronunciations commonly used for the letter 'a' spelling.
The 'ar sound' /ɑr/ begins with the tongue in the position of a 'short o' /ɑ/ sound. The tip and center of the tongue are set low, inside the bottom teeth. The top of the tongue is nearly even with the top of the bottom teeth.
To transition to the 'r' portion of the sound, the body of the tongue moves upward. The mid-section of the tongue rises so the sides of the tongue touch the middle teeth. The air travels over the body of the tongue to create the /r/ portion of the sound. (NOTE: The secondary method of producing the /r/ may be used to produce second portion of the 'ar sound' /ɑr/ instead of this technique.)