Despite the ample database of research findings on the benefits of Processing Instruction (PI), research thus has primarily made use of offline measures to establish how L2 learners comprehend and process sentences. Using online methodology, such as eye-tracking, allows research to more directly measure implicit knowledge. The sensitivity of these measures requires meticulous design choices to ensure validity and replicability. This study provides an overview of the linguistic and physical design considerations necessary for creating eye-tracking materials in SLA research. The present study demonstrates the application of these design considerations in an eye-tracking study, comparing the changes in processing patterns between two types of instruction: PI and Traditional Instruction (TI) on low intermediate L2 adult learners’ acquisition of the French imperfect aspect. The results of the experimental study show beneficial gains made by L2 learners who received PI on the French imperfect tense, this was seen in both a significant increase in accuracy scores from pre-test to post-test and change in their cognitive processing as shown by eye-movement data. The present study emphasizes the need for future studies to consider methodological reflections and key design principles in eye-tracking research.