Cut the Styrofoam balls in half. Carve them so there is a shelf that will support the edge of your speaker. Carve out a slot lower than the shelf large enough to hold the tubing. Drill a ¼-inch hole in the center of the ball for the tubing to pass through. Hot glue the speaker and tube in place in the Styrofoam ball.
Hook the speaker directly into the "headphone out" or "speaker out" on the amp. Solder a cable to the speaker. If you have a jack, that will work as well.
Place the structure in the box and surround it with foam and insulation. The talkbox will sound better if the sound passes only through the tubing.
Tape the tube to the side of your microphone stand.
Play your guitar and mouth the words directly into your microphone. Open and close your mouth to produce a wah-wah sound, and experiment to create other effects.