Simply add the mosaic image into the chart using "File - add - now choose either an ecw, a jp2 or a map (ozi) format image. Its best to remove the background water for doing this, and add it in later, alternatively you can open 2  instances of GPSMapedit and work in one - copying your work directly across to the other as you progress. Simply copy in one window, and paste in the other. It works seamlesslessly.

 You can zoom in and select areas you want to mark. In this case I am going to mark the edge of the soft mud

Now select the create object - polyline  - tool, and draw your polyline. Right click at the end of each line and select "Navico Bottom Structure"

When your editing is complete simply export to shapefile. The line will appear in your Lowrance chart, and can be activated / deactivated by using the "fishing Ground" category. This also activates bottom structure areas - see next page.

A Bottom Structure area - an Information Point could be put nearby.