How do you make "mattes"?

Be creative, use any sort of lightweight posterboard or foamboard that you can get in a dark black color and cut it to size to fit the 2 most common aspect ratios: 1.85:1 and 2.35:1.
Here's how I made mattes for my TV:
Mattes

 

Sheldon - yes, they make a surprising difference! I bought a 4'x4' piece of black, foam-core cardboard from the local arts-supply store. I measured the picture size of a letterboxed move on my 27" TV, and cut out a section that size in the center of the cardboard. I put adhesive-backed velcro on the cardboard and TV bevel, to easily attach and remove the matte.
It really helps make a more movie-like experience. My roommate thought I was a bit nuts when I was making the matte, but he is now a believer -- it just makes movies look better

 

Another vote for the two-strip model, to size 2:35 or 1.85 OARs, on a smaller tube tv.

Initially, I affixed the velcro pads to the sides of the tv, but it's a hassle since the tv is in a cabinet, tho light enough to slide out. I'm gonna switch to Patrick's model with the velcro pads on the front edges of the tv, (which in my case certainly is not a fine piece of furniture or with piano-finish.)

 

I made a set of these for my current 27" TV (until I get my 36" Wega). They do make a big difference for watching widescreen movies.

I bought some black foam-core posterboard and cut it into two strips about 5" tall and the width of my TV.

I then took some black felt and glued it to the poster board with spray on adhesive (from a fabric store). I wrapped the felt completely around the posterboard (for a more polished look) and also left about 1.5' of material hanging off of the posterboard. For the top matte, this extra material will sit on top of the TV, for the bottom matte the extra material will just hang down towards the floor.

I then made two narrow stips of felt for the sides of the TV (just to cover up the silver/wood grain/power light of the TV. I permanently affixed these strips to the top matte, and used velcro to affix them to the bottom matte.

Now, I didn't put any velcro on the TV itself, instead I just use a couple of weights (books, paper weights, etc.) to place on the top of the extra flap of material from the top matte (on top of the TV). With the rest of the "matte assembly" freely hanging down the front of the TV.

This allows me to easily slide the top matte around to get it adjusted for a particular movie, then adjust the bottom matte via the velcro. Also it is pretty quick to remove from the TV when not in use (and no messy velcro stuck to the TV). It took a bit more work and measuring to do it this way, but in the end I like the way it turned out.