Go to the thread you want to search. At the bottom you'll see a short form reading ``Search thread for....'' Fill in the box with your search query and select the ``Go'' button.
If you have multiple words in your query, it will look only for posts containing all the words listed.
The search will treat lower-case and upper-case letters the same. Each word of the query only matches to the beginning of the word. (So if your query is ``pon'' you would see posts containing the words ``Ponca'' and ``poncho'', but a post containing ``upon'' may not be included.)