Pick up a pair of dumbbells (using your legs, not your back) and stand with your feet about shoulder-width apart. Bend both your arms and knees slightly and hold the dumbbells so that your palms are facing your body. One arm at a time, raise the weight up straight out in front of you until it is slightly higher than shoulder height. Lower the weight back down over the same path and perform the same movement with the other arm. Alternate back and forth between arms until you reach muscular failure.
Tip: Make sure not to be too strict with your form on this exercise. In order to maximize muscle stimulation and prevent stress to your lower back, try to move naturally by allowing your back to lean slightly forward and back as you lift and lower the weights. Don't get sloppy with your form, but moving naturally is probably a good idea. You should also make sure not to raise the weight too high as this will shift the stress away from the front of your shoulders.