This video was shot at a metalshapers.org meet several years ago.  It shows Tom Lipton using a helve hammer to flatten out tucks and shrink the edge of a panel.  The tucks are formed by hand using a "tucking tool", which looks something like an industrial tuning fork.  The two tines straddle the panel and it's twisted left and right to put a raised ridge at the edge of the part.

Using a hammer (and a plastic or wood mallet works just as well as the helve hammer" yo flatten the tucks out.  Start at the back of the tuck, then hit either side of the tuck to keep it "locked in", then at the innermost point again.  It takes a bit of practice to know how much of a tuck you can take, how to get the most shrink from each tuck and so forth.  Needless to say, the metal should not be allowed to fold back over onto itself.