Sew a thin piece of the line fabric (aprox 1/4 inch) to the 'sky' fabric. Iron it well, pressing towards the
'sky' fabric. Stitch a second piece of 'sky' fabric to the line fabric from the back side( ie so you can see the original stitching line). Stitch as close as you like. Open it out and iron again towards the 'sky' fabric. That should do it. The line fabric should be flat, as though you did a reverse applique.
Hope you got that!