Opportunities exist to leverage mobile phones to replace or supplement in-person supervision of lay counselors. However, contextual variables, such as network connectivity and provider preferences, must be considered. Using an iterative and mixed methods approach, we co-developed implementation guidelines to support the implementation of mobile phone supervision with lay counselors and supervisors delivering a culturally adapted trauma-focused cognitive behavioral therapy in Western Kenya. Guidelines were shared and discussed with lay counselors in educational outreach visits led by supervisors. We evaluated the impact of guidelines and outreach on the acceptability, feasibility, and usability of mobile phone supervision. Guidelines were associated with significant improvements in acceptability and usability of mobile phone supervision. There was no evidence of a significant difference in feasibility. Qualitative interviews with lay counselors and supervisors contextualized how guidelines impacted acceptability and feasibility - by setting expectations for mobile phone supervision, emphasizing importance, increasing comfort, and sharing strategies to improve mobile phone supervision. Introducing and discussing co-developed implementation guidelines significantly improved the acceptability and usability of mobile phone supervision. This approach may provide a flexible and scalable model to address challenges with implementing evidence-based practices and implementation strategies in lower-resourced areas.