To accomplish the gathering and ridge forming I use two methods.  The first is the squeegy approach used to drive a shrink in from the edge of the panel.  I set the hammer up so that the dies are fairly close together and then start at the edge of the crown I want to reduce and drive inward toward the center of the crown.  If you lift up on the piece as you feed it into the dies, this tends to leave the metal flat behind the dies and gathers up the extra metal in front of the dies.  At some point a wrinkle will usually appear and the idea is to encourage it, making more passes toward the wrinkle trying to sharpen it up and trap it by running the dies up to it from each direction.  When you have captured the ridge, then gently work it down.

The other approach is similar to setting the piece with the crown to be reduced on the floor and bashing it flat with a mallet.  With the dies set up with a normal spacing, hold the piece up off the lower die a bit (perhaps 1/4") and let the upper die come down and make dents in it.  Then you gather up the wrinkles as before and work them down.  If the metal is thin, you can sometimes use light taps to gracefully work down the crown without forming wrinkles or dents first.  Rather like using a metal slapper to flatten a high spot.