BACKGROUND: Extensions to Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) and Consolidated Standards of Reporting Trials (CONSORT) reporting recommendations specifically for factorial trials have been developed by the Reporting Factorial Trials (RAFT) study group. This article describes the processes and methods used to develop the extensions. OBJECTIVE: To develop SPIRIT and CONSORT extensions for factorial trials. DESIGN AND PARTICIPANTS: A four-phase, consensus-based approach was used: phase 1: scoping review, phase 2: Delphi survey (n=104 respondents in round 1), phase 3: consensus meeting (n=15 members) and phase 4: checklist finalisation. RESULTS: In phase 1, the scoping review identified 31 reporting recommendations, which formed a long list of 50 concepts (19 applied to the SPIRIT extension and 31 applied to the CONSORT extension) to include in the guideline development. In phase 2, a three-round Delphi survey resulted in two new concepts being added and ended with 49 concepts (19 applied to SPIRIT and 30 applied to CONSORT) reaching consensus to remain, with only three concepts meeting the exclusion criteria. In phase 3, the concepts were further refined and translated into specific extension item wording, through an extensive review process conducted by the core RAFT team and leading trial experts, who attended a 2-day hybrid meeting. The resulting 9 SPIRIT items and 17 CONSORT items were further evaluated and developed through an iterative process in phase 4, to promote user acceptance and uptake. CONCLUSION: Uptake of the CONSORT and SPIRIT extensions will improve the conduct of factorial trials, as well as understanding and interpretation of such trials. By reporting on how these extensions were developed, we promote transparency of this process and share learning experiences to develop best practice when developing reporting guidelines.