ok here is the scoop. First thing you need is the tools to work with... these aren't exactly cheap, especially if you are making a lot of them. You can pick up everything that you need from www.uscomposites.com.
First thing you will need from them is the 70-25 Tin-Sil RTV Silicone Mold Rubber to make the molds of the stuff you want to cast. This stuff is very flexible and you don't need to worry about mold release agents messing up your casting acrylic. You will need to make a positive plug for your negative mold. Basically this means that if you are doing lettering, you will make all the letters raised up from the wood. This is easiest done by cutting the letters from 1/4 MDF board and supergluing them to another board in the desired look. Once you do that, make a box around the plug, and pour the silicone mold rubber into the box, following manufacturers instructions, and let it cure.
Once it is cured, you can peel away the box to release your silicone mold. Once you have the mold made it's time to break out the casting acrylic (aka liquid plexiglass) and the desired transparent tint. Here are the numbers you will be looking for... Silmar 41 Clear Polyester Casting Resin and TRA-TK001 color pigments (4 1oz. bottles of blue, red, green, and yellow)
Mix per instructions and pour into your mold. Let set up and when it's dry, you have a colored, clear piece of acrylic, which can be sanded and polished. If you want to light it up, sand it to 400 grit, and drill 2 LED holes, one on each side, assuming the piece in question is one solid piece. If the letters are seperate, you can use 1 LED per letter. Light it up and watch it glow!
It's really that simple. So why don't more people experiment with this stuff?