How do I use the [gallery] shortcode?

I don’t know why I cannot seem to figure out the simplest Codex instructions on how to apply the gallery shortcode to create a photo gallery. Can somebody please give a “For Dummies” version, what to type where, etc.? If I have 10 photos, pic1.jpg thru pic10.jpg, and have already added them to my wordpress library, how do I create a photogallery and add them to that gallery as opposed to inserting each of them to my new post one at a time as individually attached photos?