I made up this guide to answer a question on Flickr so I put it up here for anyone else who couldn't figure it out. As I recall, the Manfrotto manual (who knows where it went now) was pretty bare and it took me a bit of trying to figure it out.

I wrote more detailed steps below:

You need:
-- an alan key (hex key) that is 4mm in diameter (it should come with the kit).
-- three sections of the boom arm
-- the aluminum plates with the screw holes and screws in them

1. Pair up the plates. One plate in each pair should have no screw hole; its 'partner' has a screw that -- when tightened -- should hit its partner.

2. Fit the plates into one open section (tube).

3. Line up the screw hole of the plate with the open holes in one open section. Use the key to tighten it up. This will press the plate against the side of the open tube and also against its partner.

4. Once it is tight in the first section, fit the other ends of the twinned plates into the next section (tube).

5. Again, line up the screw holes and then tighten the screw as much as you can.

6. Test to see if you can pull the two sections apart.

7. Repeat with the third section.

*After using, keep the plates tightened in one of the sections so you don't have to do the whole thing again the next time.

Original size for printing