A wiki is a list of instructions, advice or information.
It can be edited by others, who click 'edit wiki'.
Write up instructions on you given/ volunteered topic as if you were teaching it to someone. As if you were walking him through it. It being whatever your topic is.
To include all the relevant info, go through the entire thread taking notes and adding anything new and important to your word document 'scrap paper'.
Then write it up in clear, simple language.