If two sets of underlay stitches are used, they should be placed at 90 degree angles to each other. For example, if the final fill is to be sewn at 90 degrees, the underlay would be sewn at 45 degrees and 135 degrees. It is possible to place these underlay stitches on the entire image before beginning to outline any part. However, dark colored underlay stitches will show under subsequent light colored fill. It is also acceptable to underlay each area immediately before adding the fill.
Satin stitches require underlay also.
A single line of simple running stitches up the center of a column of satin fill not more than 2mm wide will be sufficient. Satin fill columns wider than 2mm will need a running stitch on both side of the column. Another way of stabilizing satin fill is with another narrower column of satin fill stitched first and at a lower density than the final stitching. Finally, after digitizing, go back to the thread color palette, and white-out all those colors at the end that you did not use. Now you have proper colors, in proper order, with no additional unused colors on the palette.