Disconnect your speakers and plug your guitar headphones into the speaker jack instead. Your headphones will work as speakers, with the sound coming out of the side of the headphones where the cable is connected. The cable will only be on one side of the headphones.
Put one end of the rubber tube into the side of the headphones where the sound comes out. Secure the rubber tube to the side of the headphones with clear packing tape. Try to position the end of the tube directly into the center of headphone side to get right where the sound will be coming out. Completely cover that side of the headphone in the tape.
Place the headphones in a box now. A large shoe box, such as a boot box, works best for creating a talk box. The cover needs to come completely down. Depending on what type of box you are using, you may need to cut a small hole in the corner of the box for the rubber tube to come out of the box. You can do this with a utility knife.
Plug your guitar into your amp and power on the amp. Place the exposed end of the tube beside your microphone on the mic stand, and you are ready to use your talk box.