Educators struggle to select evidence-based practices (EBPs) for autistic students, which is an important skill to develop for special educators in training (SETs). To examine how these SETs learn and begin to make decisions about instructional practices for autistic students to attain their goals, we interviewed and surveyed 11 SETs before and after completing a university course on evidence-based practices for autistic students. We used the Theoretical Domains Framework (TDF), a comprehensive compilation of research-based factors that influence behaviour change, to guide qualitative and quantitative data collection and analysis to identify factors that influenced SETs' decision difficulty and intention to implement instructional plans. At the beginning of the course, most SETs described a lack of familiarity, weak intentions and surface understanding of factors. At the end of the course, SETs described and reported less difficulty in intervention planning and improved knowledge of EBP, but this was not statistically significant. We discuss important individual differences related to their difficulty, consideration of collaborative and environmental factors and intentions to implement their plans, which often corresponded with their reports of applied experience. Future directions for the pre-service preparation of special educators service autistic students are shared.