The problem with essentially teaching yourself needlepoint is that you make lots and lots of mistakes. In my early pieces, I ended my threads along diagonals (making ridges), had uneven tension, and passed threads behind lighter colors or open canvas.
Some things, such as uneven tension, improve with practice. Some things, such as ending threads can be fixed by making sure you end threads in a straight line.
But the shadow question is one that can perplex even experienced stitchers. Happily, Joni Stevenson has added a second blog post about thread shadows and strategies for fixing them.
Her tips are outstanding and easy to do. As always her close-up pictures really show the problem.
I’d just add one thing. Thread shadows can be a problem not only in open canvas but when a lighter color is between two areas of the same darker color. If the lighter color area hasn’t been stitched, the dark thread can create a shadow behind the newly stitched light color. That’s because when stitching the light color that passed dark thread is pressed up against the canvas. This means only one layer of stitching is between the viewer and the dark threads.
Always stitch the light areas first. Then when you pass the dark thread it will be behind two layers of stitching and will not show.