Begin by un-racking a barbell much like you would during a military press. Press the barbell so that it's a couple inches over your head. At this point, lower the barbell behind your head. It should now resemble a behind the neck press. Press back up so that the bar is a couple inches over your head and bring the bar back to the front military press position. This would constitute one rep.
By not locking out the weight, you're putting the stress on your shoulders and keeping it off your triceps. This is best used for high reps (8-15). You can perform these seated or standing, and usually a medium or wide grip is used. (Note: This is sometimes called a Rainbow Press.)