Open your preferred mixing application and load your unprocessed vocal recording onto an empty track. If you don't have a vocal recording to work with, record yourself singing or talking for about 10 seconds and use that. Any vocal track will do for learning the basics of vocal panning. The same technique is used in both musical and spoken-word recordings.
Layer your vocal track. In other words, create two more tracks below the first and copy the original vocal recording onto each of these new tracks. If you press "Play," the vocals will not sound any different, only louder because of the multiple layers playing simultaneously. At this point, the panning for all three tracks is centered, meaning that the tracks all occupy the aural "space" directly in front of you.
Leave the first track's panning knob centered. Think of this first track as your "lead." It will occupy the center "space" at all times and can be left alone. In most mixing situations, you will want the "space" the vocals occupy to include the center position in order to keep the vocals at the "front" of the mix.
Pan the second track all the way to the left, and the third track all the way to the right. Press "Play." Your vocal recording will now be spread across the entire panning range. This will "widen" the sound of the vocals and give them a presence that centered panning cannot reproduce. This common method of panning vocals is often called "hard panning." It should be regarded as a starting point from which you can adjust the "width" of the vocals to suit the overall mix.
Experiment with different panning settings on the second and third tracks. Sometimes panning tracks all the way to the left and right can make the vocals sound too "wide," although this panning setup is often used in professional situations. To "narrow" your vocal sound, adjust the panning knobs for the second and third tracks back toward the center position. From this point on, panning levels become a matter of taste. Adjust until you find a sound, or "width," you like.
Add an instrumental or other track to the mix, such as a drum loop, and leave its panning knob centered. Press "Play" and listen to the panned vocals in contrast to the instrumental track. Unless the instrumental track is overly loud, the vocals should dominate the mix appropriately. Mute the second and third vocal tracks and note how the vocals "drop out" of the mix because the first track and the instrumentals are both coming from the center panning "space."