While performing onstage it is difficult to hear the true sound of the main speakers. The effects and the reverb of the room can affect both the singer and the rest of the band, having stage monitors are a must. It can be confusing though to connect the stage monitors when using the same mixer as the audience speakers. There are some variances of set-up depending on the equipment that is being used.
Plug in a cable into the AUX outputs on the sound mixer. Typically the AUX jacks require quarter inch plugs. The AUX channels are separate from the main mix on the sound board, and allow you to mix the sounds to what the band needs. The number of AUX channels required will depend on the number of different mixes. There will be at least two as the vocalists will want a different mix than the instrumentalists.
Install the other end of the cable into the amplifier. If you are using powered speakers this step is not necessary. Most amplifiers have two channels on them. The number of channels needed will depend on the number of mixes that are going to be used.
Run a line from the amplifier to the speaker. Depending on the speaker, it may need either a XLR plug, 1/4" plug or an easy-lock connector. Stick with the same output from the amplifier or sound mixer. This should be a 1/4" plug. You can use either 1/4" or easy-lock depending on what is available for the speaker. The easy-lock connector is a more secure connection that ensures the line will not come out of the speaker until you want it to.
Run some tests. You will want to test the amount of amplification needed whether using an amplifier or powered speakers,
Mix the different channels by using the proper AUX dial for each input. You will need to set every AUX line for all the inputs so that the mix is set-up the way the vocalists and instrumentalists prefer.
If you must convert a 1/4" plug to XLR, do this by using a direct box. This removes the buzz from the line and improves the sound quality when converting the line.