Typically, I use a pc sync cord to trigger the main flash. As you know, the pc sync cord connects the pc socket on the camera to the pc socket on the flash.
The rest of the flashes are triggered by the main flash (wirelessly) through optical slaves. That way, when the optical slaves (on the other flashes) begin to see the main flash fire, the other flashes fire as well. That essentially happens simultaneously in what is called a short flash duration.
To work out the exposure value, I use a Sekonic L-358 hand held meter. I use the incident meter dome, and I set the meter to "flash metering" mode. To do the metering, I connect the pc sync cord from my main flash to the hand held meter (via pc sockets), and then I place the meter's dome exactly where the subject will be positioned. From the subject's position, I point the meter's dome towards the camera lens. When I trigger the meter, it fires the main flash, and the main flash simultaneously triggers the other flashes (via wireless optical slaves).
As a result the meter gives me an incident exposure value based upon all of the lighting falling upon the dome. I can use that exposure value as a starting point for determining how I want to expose the image
(e.g., shutter speed, aperture, and ISO/EI settings). Most photographers take test shots
(e.g., polaroids, instant film, etc.) to determine whether or not deviate from the recommended exposure, or they might decide to adjust the lighting scenario to better fit their vision.
Here are great videos on flash lighting and flash metering, and they might be able to give you a better understand of all of this than I can:
http://www.studiolighting.net/digital-photography-one-on-one-e002-using-a-light-meter/
http://www.studiolighting.net/digital-photography-one-on-one-e010-light-ratios
http://www.studiolighting.net/digit...g-lighting-equipment-basic-three-light-setup/