Background Extended brief interventions for alcohol misuse are effective in the general population. The process of manualising the first ever such intervention for people with mild to moderate intellectual disabilities in the UK is the focus of this study. Methods The manual was an adaptation of existing manuals based on Motivational Enhancement and Cognitive Behaviour Therapy and was used in a feasibility randomized controlled trial, the EBI‐LD study. The sessions were recorded and scored using an adapted version of the Yale Adherence and Competence Scale (YACS II). Feedback was provided by therapists. The trial is closed. Registered: isrctn.com; ISRCTN58783633. Results The quality of the sessions provided was rated as good. Therapists were able to cover all topics within each session. Main challenges included session duration and homework task completion. Conclusions We recommend the duration of the sessions to be extended to 40 min to accommodate carers in the session and to enhance their support in homework task completion.