According to ganesh shastras, it is given that his "mantra japa" should be done while eating fruits or laddu (a round shaped sweet).
To get siddhi or fruit, one should recite his mantra for 16,000 times.Below is the vedic Ganesh mantra:
Asyashree vedic ganesh mantrasya Sughora rishihi nibhida gayathree Chandhaha Vedic Ganapathirdhevata mama sarvabheesta sidhyarthe jape viniyogaha
Om Sughora rishaye namaha shirasi
Om nibhida gayathree chandhase namaha mukhe
Om vedic ganapathaye namaha hrudhaye
Viniyogaya namaha Sarvange
Om ekdantaya angustabhyam namaha
Om vidhimahe tharjaneebhyam swaha
Om vakrtundaya madhyamabhyam vashat
Om dhimahai anamikabhyam hoom
Om tannauo danti karathalakara prastabhyam phat
Om ekdantaya hrudhayaya namaha
Om vidhimahae shirase swaha
Om vakratundaya shikhayai vashat
Om tannauo kavachaya hum
Om danti nethrathrayaya vaushat
Om prechodayat asthraya phat
Rakthamoorthim ganeshancha sarvabharana bhooshithamRakthavasthram threenethram cha rakthapadmasane sthithamChaturbhujam mahakayam dhvidantam susmithananamIstancha dakshinehasthe dantham cha dhadhathaha karePashankushou cha hasthabhyam jatamandala vestithamLalate chandrarekhadyam sarvalankara bhooshitam
Then start jaap Vedic Ganpati Mantra
"Om ekdantaya vidhimahe vakratundaya dhimahi tannauo danti prachodayat"