Gather your material. Whether you've done movies, TV, commercials or filmed yourself doing a monologue, gather every piece of footage you have of yourself on tape.
Narrow down the footage. A reel should show a range. If you have ten scenes of you getting angry, then only one is valid. Agents and casting directors want to see that you can act in a range. That you can tap into a myriad of tools and emotions. Look at your work and narrow it down to specifics, choosing one of each type of scene.
Hire an editor. If you're tech savvy, you can save yourself some money doing it that way by purchasing an editing program which you can load to your computer. But an editor provides something way more than a computer ever can: a discerning eye. Most likely he has cut many reels and has a better idea about what "works" and what doesn't. Also, most people who are editors for a living possess professional equipment that will deliver a superior product.
Stop being in love with yourself. Six minutes or five scenes is the MOST you ever want to show of yourself.
Add titles and music. While your reel shouldn't include a montage sequence of you posing for the camera with today's hippest song, the beginning and end should be bookended by your name with some sort of music underneath. Your editor should be able to help set the mood depending on the content of your reel.
Show everyone. Well, not everyone! But now that you have your reel, do not hesitate to send it to agencies, casting directors, or agents.