Purchase a color organ kit from an Internet retailer, such as Electric Goldmine, Centerpointe Electronics or ElectronicKits.com. Included with the kit you will find a circuit schematic and electric components. If you have no experience reading circuit schematics, there are various resources explaining what the symbols mean, such as ArtSites.ucsc.edu.
Insert your components into the printed circuit board included in your kit, bending the connectors to keep the components in place. Orient the components as displayed in your circuit schematic and as indicated on the printed circuit board.
Solder your components into place by heating the connectors with the tip of your 25-watt soldering iron at the joint of the circuit board and the connector for one to two seconds. Once the component connector is heated, feed a small amount of your rosin-core solder between the head of your soldering iron and the joint. Repeat this process for all component joints.
Alter your project box to accommodate the color organ by using a hand-held rotary power tool to drill and shape holes for your external components. Drill a small circular hole for your audio input, a separate hole for your power switch, a small hole for your DC power input and a rectangular hole the size of a plug head for your DC power supply output.
Place the audio input and power switch of your color organ on the inside of your project box and fix both components firmly in place by twisting the included bolts from the outside. Thread your DC power-in cord through the hole and fix your DC power into the rectangle. Close the circuit within the project box.