Miko: In Japanese, it means "priestess". Normally talks about the girls and women who work and serve at the Shinto shrines. But the authoress of the manga "Fushigi Yuugi" (Mysterious Game), Yuu Watase, took the Miko concept and interpretated it as the girl who has the mission of directing and organizing the summon of a god. That's why the main character of the story, a girl named Miaka, is called Suzaku no Miko (Suzaku Priestess): she was chosen to summon the Protector God of Konan Empire, Suzaku.
From this the "miko-hood" here in the net would be a mixture of those two concepts: a Net-Miko is a fangirl who takes the task of (instead of "summoning") directing the cult to the bishonen, bishojo, pairing, or fandoms she's a Miko of. Each Miko would have 7 Seishis.
taken from There is Something about MitKo by Minako.
Basic Guidelines to Becoming a Miko
1. The position must not be taken.
2. Please declare youself pending at the SlamDunk Babble Club
3. Consult other related Mikos in existence if necessary.
4. Be willing to take on responsibility for the pairing/character/alternate and not for the title only.
5. Produces an amount of fanwork.
6. Gain the approval of everyone by proving yourself.
Seishi: The term most likely orginating from Sailor Moon's 'Senshi', it briefly means 'Star Warrior'. A Seishi is supposed to help a Miko in her duties and responsibilities. Contributing to the character/pairing/alternate is the main criteria. Pending Seishis should consult the respective Miko in existence.