Karthik Purnima Puja Vidhi

Karthik purnima also known as Dev Diwali- the Diwali of Gods it is also sometimes called as Tripuri Purnima or tripurari Purnima.Actually 7 main reason for Karthik Purnima Festival :

1.  In some legends of Kartik Poornima, the term is used to denote the three demon sons of Tarakasur. Tripurari is an epithet of God SHIVA. SHIVA in his form as Tripurantaka ("Killer of Tripurasura") killed Tripurasura on this day.This day is also called "Dev Diwali". Diwali is the Hindu festival of lights.
2.  Kartik poornima is also the birthday of Matsya, god Vishnu's fish-incarnation avatar. 
3. It is also the birthday of Vrinda, the incarnation of the Tulsi plant 
4.  This day also is considered special for Radha, the lover of Krishna - Vishnu's incarnation. It is believed that Krishna and Radha danced rasa and Krishna worshipped Radha on this day. 
5. This day is also dedicated to the pitrs(Deva Pitrah), dead ancestors. 
6. The festival has even more significance when the day falls in the nakshatra (lunar mansion) Krittika and is then called Maha Karthik. The nakshatra is Bharani, the results are stated to be special. If it is Rohini nakshatra, then the fruitful results are even much more.It is believes that day is supposed to bring benefits and blessings equal to the performing of ten yajnas (sacrifices).
7.  And the Most Important and Most significance reason is Lord Kartikeya, the god of war and son of SHIVA's birthday.(Some sources also mention that Lord Karthikeya's get that name because of the he was born on Karthik Month.)

Believes


Kartik Poornima is closely associated with Prabodhini Ekadashi which marks the end of Chaturmas, a four-month period when Vishnu is believed to sleep. Prabodhini Ekadashi signifies the awakening of the god. Chaturmas penance ends on this day. Many fairs that begin on Prabodhini Ekadashi end on Kartik Poornima, Kartik Poornima usually being the most important day of the fair that happen in some of the region in INDIA.Kartik Poornima is also the last day to perform Tulsi Vivah ceremony that can be performed from Prabodhini Ekadashi.
Also, it is believed that Vishnu, on this day, returns to his abode after completing his stay in Bali. Hence, the day is known as Deva Diwali.

 A ritual bath at a tirtha (a sacred water body like a lake or river) at a pilgrimage centre is prescribed on Kartik Poornima. This holy bath is known as "Kartik snana".An holy bath at Pushkar or in the Ganges river, especially at Varanasi is deemed as most auspicious. Kartik Poornima is the most popular day for bathing in the Ganges at Varanasi.

Annakuta, an offering of food to the deities, is held in temples.People who have taken vows on Ashwin full moon day, end them on Kartik Poornima. God Vishnu is also worshipped on this day. Any form of violence (himsa) is prohibited on this day. This includes shaving, hair-cutting, cutting of trees, plucking of fruits and flowers, cutting of crops and even, sexual union.Charity especially donation of cows, feeding of Brahmins, fasting are religious activities prescribed for Kartik Poornima. Giving gift of gold is said to fulfill all desires of people.

 Tripuri Poornima is only celebrate next to Mahashivratri, amongst festivals dedicated to Shiva worship. To commemorate the killing of Tripurasura, images of Shiva are carried in procession. Temple complexes in southern India are lit up throughout the night. Deepmalas or towers of lights are illuminated in temples. People place 360 or 720 wicks in temples, to secure escape reaching hell after death.The 720 wicks symbolizes the 360 days and nights of the Hindu calendar. In Varanasi, the ghats come alive with thousands of diyas (deepam) (brightly lit earthen lamps).People gift lamps to priests. The lamps are kept throughout the night in houses and Shiva temples. This day is also known as "Kartik Diparatna" - the jewel of lamps in Kartik.Lights are also floated in miniature boats in rivers. Lights are placed under Tulsi, Sacred fig and Amla trees. The lights in the water and under trees are believed to help fishes, insects and birds who saw the light to attain salvation.

PUJA VIDHI

1.  During the last 5 days of  Karthik Purnima (panchaka) women take pure bath and draw beautiful kolam (flower design) in their house.The devotees also take a bath in the evening during moonrise and offer worship by way of six prayers such as Shiva sambuti, Satait and so forth.{ It is believed that all gods come to earth on this day and bless the peoples who offered a prayer.}It is also known as Karthik Snana.
2. Devotees also may involves in donation,charity and daan(dharma) and etc due to so much benefit on the special day.{People those who donates on this day is blesses with ten religious sacrifies (yagya).}
3. Prayers presented on every Purnima starting from Karthik Purnima will offer positive results.
4. Also some sources mention that donating the lit become very auspicious on this day.
5.  An offering to the gods known as Annakuta is held in the holy places. It is greatly believed that donations, charities,daan and etc during Poornima is equal to 100 Ashvamedh Yagyas and help achieving salvation. This might helps in getting near to SHIVA Pa and blessing for well prosperity and peace life for devotess. 
6. Some sources mention that taking vrat starting 5 days before and end on the Karthik Purnima is great and helpful way of worshiping LORD KARTHIKEYA
7. SHIVA Pa temples are commonly visited on these days.
8. Lighting some lits on the house may give precious happiness,peace and prosperity.

Note: Will update time to time after collect much information about Karthik Purnima.