The Purnima fast is fruitless without Udyapan; the exact rules are given in the Bhavishya Purana.:Purnima fast is observed every month.

News Topical, Digital Desk : According to Hindu scriptures, any fast is considered complete only when it has been properly performed. The Purnima fast is considered a very important date in religion, and many devotees observe a fast on it.

Fasting on the full moon day brings the mind a moon-like calm. If you also observe the full moon fast, be sure to perform the Udyapan (the end of the fast) using the method described in the Bhavishya Purana, so that you receive the full merit of the fast.

Prepare for Udyapan in this way

Get up early in the morning, clean the house and take a bath etc.
If possible, plaster the ground of the place of worship with cow dung or you can also sprinkle Ganga water.
After this, make a 'Chowk' at the place of worship and install an earthen pot in the middle of it.
Fill the Kalash with water, betel nut, turmeric lump, and rice and make a Swastika on it and place mango leaves on top.
Now install the idol or picture of Lord Vishnu and Goddess Lakshmi and bathe them with Panchamrit.
Offer Kalava, sandalwood, flowers and garland during the puja.
Listen to the Purnima Vrat Katha and offer rice kheer to the Lord.
At the end, distribute Prasad to all the people.

Definitely do this work

Havan must be performed during the completion of Purnima fast.
Feed the Brahmins and send them off respectfully by giving them donations as per your faith.
You can donate white items like rice, sugar, or milk on this day.
At night, offer arghya to the moon by mixing raw milk and water.
One should not sleep on the night of Udyapan, rather one should stay awake the whole night singing bhajans and kirtans.

Pay special attention to these things

According to the Bhavishya Purana, the full moon day of the month of Jyeshtha is considered the best for performing the Udyapan (consecration) of the Purnima fast. However, you can perform the Udyapan on the full moon day of any other holy month.
The person observing the fast should eat only after the Brahmin and the guest have finished their meal.
During Udyapan, do not have any feeling of hatred towards anyone and follow celibacy.