Making a custom field searchable in search.php

[Arkham]

I have an extra field for threads ('field1') that isn't being searched by default. How can I add a field to search.php in order to have it searched?

(The "custom field" isn't a user profile custom field, but a field added to the thread table called 'field1'. Each thread has something specified for this field.)

I think I have to add something to the query, but my knowledge of MySQL is pretty limited.

Thx.