Adaptation of guidelines for use at the national or local level can facilitate their implementation. We developed and evaluated an adaptation process in adherence with standards for trustworthy guidelines and the Grading of Recommendations Assessment, Development and Evaluation (GRADE) methodology, aiming for efficiency and transparency. This article is the first in a series describing our adaptation of Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines for a Norwegian setting.