Many students struggle with constructing mathematical models. Because instructional videos can help students learn the steps involved in mathematical modelling, we qualitatively investigated how four pairs of upper-secondary students (Mage = 16.63) in a laboratory setting used an instructional video for constructing a mathematical model and whether watching the video qualitatively changed their model construction. From a qualitative text analysis, we found that students used the instructional video to structure their solution process and to perform the steps understanding, simplifying, and mathematising on a subsequent modelling problem. However, since video use did not always result in qualitative changes to the constructed mathematical model, support in classroom settings will still be necessary.