Wake up in the morning on the day of Janmashtami, take bath, and observe fast for the whole day.
Bathe the idol of Krishna before worshipping with Gangajal and milk.
Make him (the idol) wear new clothes and adorn him with a peacock crown, flute, Vaijayanti garland, Tulsi Dal, Kundal, etc.
Decorate the swing of Lord Krishna with flowers and other decorative items.
Offer him fruits, flowers, makhan (butter), sweets, nuts, etc, to Lord Krishna while worshipping and light incense lamps.
Worship Shri Krishna at midnight too, and swing him on a swing and perform aarti.
Distribute the prasad once you concluded the worship.
NOTE: According to the Drik Panchang, no grains should be consumed during Janmashtami fasting until the fast is broken on the next day after Sunrise. All rules followed during Ekadhashi fasting should be followed during Janmashtami fasting also.