I'd had this dimmer switch laying around for quite some time. Mainly, because I wasn't sure how to install it. I opened the box, and saw this:
Two red wires, one green and one black. OK. Fine. I just had to look up which connected to the existing wires in the box.
So, this is how you do it.
Cut the power at the circuit box!!!
Take your cover plate of the existing switch:
Next, remove the screws on the top and bottom to loosen it so you can get to the wires in the back. Pull the unit out a little bit to help you see.
Remove the existing switch by removing the screws and taking the wires out.
Attach the new dimmer wires to the existing wires by twisting on those red caps that came with the new dimmer.
Black wire connects to black wire. Red to red. Other red to white, and the green wire is your grounder so that connects to the copper wire. It will end up looking like this:
Next, jam all the wires back into the outlet box and screw the new plate in. Yes, "jam" is a very technical term. It might take a little fidgeting to do this - take your time.
Add the top cover plate. Step back and look at your work. Pat yourself on the back.
Turn the electric back on and test the switch (make sure the "dimmer bar is all the way up at first so the light turns on). Oooh and Aaah over your talents.
You may need to touch up the paint around the new cover plate.
Tada! I heart dimmer switches.