When you are using the weed wacker, the wacker will either pull the grass clippings toward the grass area or push them toward the mulch (depending on whether you cut on the left or cut on the right sie of the head). Trim the grass so the cut grass goes toward the grass area. Trim first then use the lawn mower to cut the grass. The mower will pick up the trimmings from the wacker.

Thank you garden-guy! Sounds pretty simple. With all of the rain here the grass is growing faster than we can keep up with it.