Go around each block, inside and out. a Serpentine (squiggly) line would work if you free motion it...but even just using your zig zag on a medium width, with a long stitch will work ok. Use the zig zag that is small stitches, not just a zig and a zag...That way you can use a walking foot to do the stitching. I'd use a matching thread or invisible thread.