I would recommend that you stitch in the ditch on the sewing lines using a walking foot if you have one. If you don't have one just do all your straight lines first to stabilize the sandwich.
If you want to do a heart inside of the stitched squares, you can either FMQ or regular stitch them. FMQ with the feed dogs down (FMQ foot on) will let you move the quilt but stitch the heart shape without having to turn the quilt. If this isn't working for you, you can do a regular straight stitch (regular foot) heart by turning the quilt as you stitch the heart. Since you have already secured the quilt with the stitch in the ditch grid, you should be able to stitch the hearts.