Assuming you want the lighting to come entirely from the flash(es), proper exposure will be determined by the output of the flash(es) and the f stop setting. Set the shutter speed high enough so that ambient light does not influence the exposure. If you need a particular DOF, set the appropriate f stop and then vary the output of the flash(es) and/or move them closer to or further away from the subject. If you want to show some of the ambient light in the photo, change the shutter speed to get the effect that you want. An incident flash meter can be a big help, at least until you get used to the system.