Oh, it's easy --

You'll get a piece of bamboo that's thick & curved at the ends, and flattens & widens to its center. Fold it so the curved ends enclose a small metal tube with cork on it, then start wrapping them together with string & glue. Then, go to the thin, folded end of the reed, and trim it off to create the opening for air to go through. Now, lightly shave & shape the flat outside surfaces, making some areas thinner and leaving thickness in others to give some structural support to manage the buzz of the thin areas. Work it until it gets a good "crow" -- buzzing a pitch without being in an oboe -- at about a concert A (I think). If you can't get it to buzz, and/or you've trimmed & cut it so much that it buzzes above an A, it's FUBARed and you need to start with a new one.

Well, that was all from memory, going back to flute-double reed class in college. A bassoon reed is almost the same, except that it's broader and doesn't have the metal tube & cork.

Here's a how-to. It goes into a LOT more detail than I cared to remember.
http://everything2.com/index.pl?node_id=1295338