Unplug your amplifier.
Lay your amp down with its grille facing the ground. Do this on carpet.
Unscrew the back panel of your amp. It should be anchored with four or six screws in the corners Remove the panel and set it aside.
Look at the top of the inside of your amp. Locate the base of your 1/4-inch mono guitar input.
Plug in your soldering iron. You will need to let it heat up for at least 10 minutes.
Touch your soldering iron to the wire connections on your amp's input. This should melt the solder so that you don't have to cut the connections.
Unscrew the ring screw at the top of your 1/4-inch mono connection. Use your pliers for this. Turn the screw counter-clockwise until your jack is free. Remove the jack from your amp.
Remove your RCA jack from its package. Look at the jack. The jack will have a removable tip. This tip functions the same way as the ring screw. The jack will also have two contacts just like the 1/4-inch jack you removed.
Remove the tip from your RCA jack by twisting it counter-clockwise. You may need to do this a few times before it is entirely off.
Slide your RCA connector up under the top of your amp so that the jack is exposed. Place the tip on the top threading and tighten the connection by turning the ring screw clockwise. You may do this with your hand.
Melt some solder on the tip of your iron. A very small bead should do.
Touch one of your amp sound wires to one of the contacts and transfer your solder to this connection.
Solder the remaining prong using the remaining wire. It does not matter what order or wire is used with this.
Plug your audio device into the RCA input.
Plug in your amp. Turn it on.
Play audio from your device. You should hear it coming out of the amp.