The first option is to raise the back of the tongue so that the sides of the tongue touch the back teeth. The center of the back of the tongue is lower and the air travels through this groove to create the sound.
Alternatively, the tip of the tongue can be raised and curled back behind the tooth ridge while the back of the tongue stays low. The air still travels over the back of the tongue, but moves more along the sides and tip.
The tip of the tongue never touches the tooth ridge during the American English 'schwa+r' /ɚ/ or 'r sound' /r/.