This study investigated whether implementation intentions could be a promising metacognitive strategy for self-regulated learning, helping students to develop beneficial study routines by planning when and where to study. In a 42-day micro-randomized trial, we examined the effectiveness of a mobile intervention with the goal to increase the likelihood of children studying with a vocabulary learning app. Children (N = 357) received mobile prompts in the evening asking them to formulate appropriate implementation intentions (i.e., if-then plans specifying when and where children would like to study; “When I finished brushing my teeth, then I will study vocabulary.”) for the next day to increase the likelihood of studying vocabulary. In addition, ambulatory assessment was used to capture children’s daily fluctuations in motivation. These findings suggest that well-formulated if-then plans are a useful metacognitive strategy for action control. For children to plan their own studying, researchers and teachers need to ensure that they explain it carefully, provide examples of high-quality plans, and preferably scaffold the creation of plans by providing feedback.