People who know what they're talking about.
Michelle Craske and David Barlow are renowned experts in Generalized Anxiety Disorder (GAD). They developed a proven step-by-step program to overcome GAD at the Boston University Center for Anxiety & Related Disorders.
Mastery of Your Anxiety and Worry is a self-help workbook that is best used in consultation with a therapist or anxiety coach, but can be used on your own.
To find a therapist or coach, check out this article on ways to find help (in-person and virtual).