Performing Sashtiapthapoorthi, the 60th birthday pooja or marriage ceremony, involves the following steps:

1. Planning and Preparations: Plan the event and fix a suitable date and venue for the ceremony. Make necessary arrangements for the venue, decorations, and items required for the rituals. Involve family members and close relatives in the planning process.

2. Rituals and Prayers: Begin the ceremony by performing traditional rituals and prayers. Seek the blessings of deities and ancestors for the couple's well-being, longevity, and a prosperous married life. Offer flowers, fruits, incense, and other sacred items as part of the rituals.

3. Vedic Mantras and Havan: Chant Vedic mantras and hymns specific to Sashtiapthapoorthi. Conduct a havan (fire ritual) where offerings are made to the sacred fire while reciting mantras. The havan symbolizes purification, auspiciousness, and the divine presence.

4. Exchange of Garlands and Blessings: The couple exchanges garlands as a symbol of love and mutual respect. Family members and well-wishers bless the couple, expressing their wishes for a blissful and harmonious married life.

5. Feasting and Celebrations: Conclude the ceremony with a festive meal or feast for all the guests. This is an opportunity for everyone to come together, celebrate the couple's milestone, and share joyful moments.

It is important to note that specific rituals and practices of Sashtiapthapoorthi may vary based on regional customs, cultural traditions, and family preferences. Consulting with a priest or individuals experienced in conducting such ceremonies will provide more accurate guidance and instructions based on your specific cultural and religious background.