If you want to add a stitch, it is advisable to do so one-at-a-time. Otherwise you have to be careful to knit the stitches very tightly. In order to add a stitch knit/purl one stitch but do NOT remove it from the left needle. Then purl/knit the same stitch. Then take the stitch from your left needle. Done!

You add width to your work if you add stitches. It is useful to add stitches regularly throughout a row. If you have a rather complicated pattern, try to add stitches to the second and the last but one stitch. This way you do not disturb your pattern.