Count the number of signals you need. For a duo with two vocals and two guitars, you'll need four individual signals. Two microphone cables will connect to the signals from the vocal microphones, and two will connect to the signals from the guitars.
Connect the male XLR ends of the cable into the mixer. You'll need four separate inputs on the mixer for a duo, two for vocals and two for instruments. The XLR ends of stage snakes are commonly labeled with numbers. Note the number on the XLR and the input number where you plug it in. For example, plug the male end of the XLR cable numbered "1" into the input of the mixer labeled "1." Keeping track of the XLR number and the input number on the mixer will allow you to make the proper XLR connection on the stage.
Connect the female XLR ends to the onstage vocal signals. Female XLRs will connect directly into vocal microphones. If the input on the mixer for microphone "1" is connected to the XLR cable "1," then connect the XLR female end numbered "1" to the first vocal microphone. Do this for the second vocal microphone as well.
Plug the direct boxes into the proper XLR cable. Direct boxes are commonly used to connect guitars to the mixer without the need for a long cable. Connect the proper female XLR from the stage snake to the male of the direct box. Two direct boxes will be needed for a duo, one for each guitar. Again, note the number of the XLR cable so that you can properly mix each individual signal.
Mix each individual signal and make sure each signal works.