This is quite simple, first you use the Hi temp silicone to seal the metal to plastic junction of the iron. Leave that to set overnight.
Cut off about an inch of the 3mm brass tubing, try not to pinch it closed. If you do use the soldering iron tip to force it open again. I used a dremel cut off blade to slice off 1 " of the tubing. If you got the tubing assortment which is more useful you can select a 2-3mm diameter piece to work with.
Use a flush wire cutter to snip about a tenth of an inch into one tubing end, do this 4 times at about 90 degrees offset each time and then open out the snipped tubing end so it can 'stand' up.
Slide the 4-40 washers over the 1" 3mm tubing, you may need to bore out the washers a little. I used a dremel conical grindstone to do it.
This 1" tubing and washer component simply replaces the soldering iron tip to direct the air. It is easy to swap back and forth like any other soldering tip.
Now hold the soldering pencil as you would when working and note where the pad of your thumb's top joint sits near the edge of the rubber insulation sleeve. Mark this spot and carefully drill a 1/8" hole into the iron plastic handle, about 1/4" deep, to suit the leg of the plastic air TEE. You should snip off the barb part of the Tee leg that holds the tubing. Use a bit of the silicone to affix the Tee leg into place so that your thumb easily throttles the open end of the Tee. Attach the air tubing to the TEE and the air pump .
Once the silicone has cured you can switch on the air pump and the iron at max temp. The new Iron will smoke a bit when hot. It stops after a while.
Now try out your soldering technique by applying hot air, using your thumb to throttle the air flow. It's quite convenient. I have operated my iron for 3 years like this and soldered thousands of parts!
As an improvement I opened the soldering iron control base and wired the air pump directly to the iron controls for a single switch operation. Also I used a bit of hookup wire as a twist tie tubing clamp on the Tee. Finally, using hookup wire twisted loops to guide the air tubing along the iron's electric cord keeps things neater when working.
I'll post some pics if there's interest!