This is a A/B/Y box. It acts as a switch. You can plug in a guitar and have it's output go into two amplifiers.
You can choose between the amplifiers or use both at once.
This is a very simple build. As a beginner I had only moderate work.
Step 1: Bill of Materials
2x 5mm LEDs
2x 4,7k Ohm Resistors
2x DPDT Switches (http://www.banzaieffects.com/files/images_detailed/d_4217.gif)
3x 1/4 jacks (P-10 jacks - http://www.effectsconnection.com/oscommerce/images/marshallstylemono.jpg)
9v battery clip (http://www.amqrp.org/kits/NCKeyer/elmer/P7190015a.jpg)
Something that can drill, such as a Dremell
Soldering Iron & Solder