Jai Srimannarayana!

Happy to know about Sitaramakalyana Mahotsavam in your home.
1.Perform Vishwaksena Puja
2.Conduct Punyaha Va:chanam
3.Arrange a place for Kalyanam. Take Sri Rama & Sita to that mandapam.
4.Provide Asanam to Lord RAma.
5.Offer Madhu Parkam, ie mixture of Yogurt & Honey.
6.Decorate Him with a New Yajno:pavi:tham.
7.Conduct mangala ashtakam chanting.
8.According to the local tradition, any other offerings like Gudam Jeerakam mixture may be offered.
9.Perform PUja to Mangalasutram and offer to Sita after touching to Rama’s hand.
10.Akshatas may be sanctified and offered to both by each one’s hand. She offers Him and He offers to her.
11.Brahma Grandhi bandhanam and Vedic asi:rwa:dam.
12.Chant Va:rana Ma:yiram and give Ha:rathi.
13.Distribute Prasadam etc., to the devotees.

There are the main activities. You may chant manthras to each of these
activities at length.
Contact any pandit to get the manthras.