Caveat:
First, these instructions are based on those given on the Astro-Physics web site. Second, my construction method may well not be the best one. Use mine for possible ideas, but bear in mind that I'm newer at this than are the guys at Astro-Physics.

Materials and Prices:
The first step, without which nothing else makes any sense, is to obtain some of the Baader Solar Film. There are other materials but this is the only one I am familiar with; the Astro-Physics site has complete information on why the Baader material is reputed to be the best. Astro-Physics sells two sizes of sheet with a really bizzare price structure, an average of $3 or $11 per square inch, depending on the size of the sheet (and the larger sheet has the higher average price!). I privately purchased a sheet 6" x 9" for 30 cents/sq inch ($16.20, total) from one of the regulars on Usenet group sci.astro.ameteur; I planned to cut my film into 3" x 3" pieces, meaning that, for each filter I make, the film will cost $2.70. The round cardboard box came from JoAnn Fabrics, an American chain of craft stores (it has an embossed design on the outside, but this portion will be removed and discarded); the 10" size cost $7. I also used double-sided tape and glue sticks for an additional (approx) $3.

Construction:
1. The box lid will serve as the filter's endcap, and is just fine as it is. For now, though, we need to use it as a cutting guide. Place the lid onto the bottom of the box, then run a utility knife around the outside of the box, using the edge of the top as your guide. When finished, the box bottom will fit exactly into the box lid.

2. Place what's left of the box sides into the upturned box lid, and again run your utility knife around the outside. This will give you a ring of cardboard the same height as both the box lid and the box bottom. Cut this ring just once, so that it forms a strip. Place the box lid (now the "endcap" somewhere where you won't mistake it for the box bottom (now the "mask").

3. Using a glue stick, completely coat the inside of the mask side (*not* the inner face of the mask, though) . Take the cardboard strip and place it inside the mask, then expand it so it comes into contact everywhere with the inside of the mask side, and hold in place with clothespins. You'll have to trim some overlap. This strip forms a liner and sizer for the inside of the mask sides, and you may have to add more strips to make the inner diameter of the mask snug enough that the mask will remain on your telescope when pointed at the ground (this is for eye safety).

4. When the glue is dry, remove the clothespins and turn the mask face up. Figure out where you want the opening to go, and how big it should be (I used the endcap of my telescope, which is its own off-axis mask, for a template). Very carefully, cut the hole.

5. (This step is optional. I did it as a retrofit when I became concerned, perhaps needlessly, that the sunlight could warm the mask enough to soften the interior glue and tape.) Coat the entire outside of the mask and sides with the glue stick, then affix aluminum foil, shiny side out, smoothing it out as you go. I did this in two steps, one piece of foil for the face and one for the side; how you do it is up to you.

6. Using any spare piece of cardboard or posterboard that might be lying about (the back of a writing tablet would be ideal), cut a round piece that will fit into the inside of the filter. This will be the "inner mask" and the Baader film will be sandwiched between the inside of the mask face, and the inner mask. Cut a hole in the inner mask that matches up with the one in the mask. For ease of removal if you have to replace the film, the inner mask doesn't need to completely cover the inner face.

7. Now place double-sided tape all the way around the aperature hole of the mask, on the *inner* face, as close to the hole as possible. Carefully lay down a precut piece of film over the hole, large enough that there is at least 1/2" of film all around the hole's edge. As per the Astro-Physics instructions, the film should lie flat over the hole without any stretching or tension, whatsoever. Now apply double-sided tape around the aperature hole of the inner mask, and carefully lay it down over the film (you should be able to make out and match up with the hole in the mask). Firmly press the inner mask and mask together, avoiding touching the film. The film is now sandwiched between the two masks, and taped to each.

One more thing: for storage, my wife let me have a 12" tupperware "pie safe," inside of which I taped a couple of those dessicating packets that come packaged with electronics. I place the filter, inside its endcap, face up inside the pie safe and, with the lid properly fastened, it's protected from virtually any damage.

Warning: Remember to follow all safety precautions each and every time you begin an observing session. Always test the filter in place before pointing it at the sun, never try to use your finderscope to aim and, most important, if anything about your filter looks odd to you, don't use it!!!
 

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