Coincidentally, I happened to have built one earlier this morning!
First, you need to get a funnel from the auto supply store. Most funnels won't work. The neck needs to be wide enough to accomodate an eyepiece from your telescope. A Blitz Super Funnel (#05034) works perfectly. I couldn't find one at any of the auto shops, but found one at KMart.
You also need a couple of hose clamps - a small one (1.5 inch) and a large one (5 inch).
The one thing you will need to order is the rear-surface projection screen. I ordered a Da-Lite High-Contrast Da-Tex rear-surface screen #95774. I bought two pieces, each one-square foot in size, but you only need one (I assume problems will happen, hence the duplication).
Step one, saw the funnel neck -- about ten inches from the large end, and seven inches from the small end.
Sand the rough areas.
Cut into the small end, about an inch or two deep in two places - this will allow a hose clamp to sqeeze this end and secure the eyepiece.
Insert the eyepiece of your choice - I used a 12.5 mm.
Use the small hose clamp to secure the eyepiece in place.
Wrap the rear-projection screen fabric on the large end and secure it with the larger hose clamp. Either side of the fabric is as good as the other and it doesn't matter which side faces in or out.
It is best to use the solar funnel with a refractor, but I don't have one - so I used a reflector. The problem with using a reflector to look at the sun is that the mirrors can be damaged. Therefore, one has to reduce the amount of light going into the scope. I drilled a 1.5 inch hole into the end cap used in storing the telescope, but this seemed to reduce too much of the light. I ended up lifting the cap and holding it slightly ajar to allow more light to enter.
I didn't read the instructions in Sky and Telescope, but I assume they are sufficient.
It may not be visible in this image, but the projection does so sunspots.
Is it worth building one of these? It is not as good as a direct view I get with the Questar or the Coronado PST, but it is sufficient for public viewing where two or more are gathered together.