Take an ordinary drinking straw and cut off any pleated elbow ends. This generally works well when the straw is cut down to a 4 to 6" length.
Step 2: Flatten an end
Take one end of the straw and flatten the last half inch of it. I find that the blunt edge of a butter knife rubbed against the end of a straw works well. Make sure the creases are well defined, but don't flatten too much or it will become difficult to blow. For those who are familiar with Oboes, you'll probably start to recognize what we're doing.
Step 3: Cut a reed
On the flattened end cut off the last quarter to 3/8" of the EDGES. Angle the cuts inward toward the flat end of the straw so that it forms an angled U.