The ArcScan extension is only active in an edit session. The Start Editing command enables you to begin an edit session.

In order to trace features from rasters, you need to tell ArcScan the width of the patches of cells that will be considered lines in the image. You set raster snapping options to define this width, as well as the size of solid patches of raster to snap to, the maximum size of holes to ignore, and the foreground and background colors. These settings influence the behavior of the tracing. You can set them in the Raster Snapping Options dialog box.

Once you've set the raster snapping options, you can turn on snapping to parts of the raster in the Editor Snapping window. In this example you turn on snapping to centerlines and intersections detected in the raster.