In the morning after taking bath, offer prayers to Lord Ganesha. Ligh a "Deepak" (earthen lamp) and do any prayer or Aarti of Ganesh ji.
Ideally, remain on water throughout the day, and chant the name of Ganesha with full devotion if you want to keep a complete Sankashti Fast. However, those people observing partial fast can eat Sabudana Khichadi or fruits, nuts or curd rice. Salt must be avoided.
Sankashti Chaturthi Evening Puja
Perform Pooja after sighting the moon.
After sighting the moon, an idol or photo of Lord Ganesh is decorated using flowers, and after lighting a lamp, coconut, banana etc are offered to Lord Ganesha. Prepare "Prasad" with any sweet you like and perform aarti. You can also chant mantras, light incense etc and after the pooja is complete, distribute and eat the Prasad.
The importance of Sankata Chaturti Vrata was narrated by Lord Krishna to Yudhishtira, the eldest among the Pandavas. The importance of it is also narrated in the in the Narasimha Purana and Bhavishaya Purana.