The Vakratunda Mahakaya - वक्रतुण्ड महाकाया Ganesh Mantra is probably known to most Ganesh Bhakts. This Mantra should be chanted just one with full sincerity and devotion, visualizing Ganesha in your mind as being infinitely radiant and glowing more brilliantly and brightly than a Million Suns.
After this, the practitioner should chant he second two words Ganpati Mantra for One Mala [any counting Mala can be used]. You will notice that I have omitted the Om ॐ at the beginning of the Mantra. However, if you feel more comfortable with the Om or are already practicing this Mantra, then you can chant it with the Om at the beginning of the Mantra.