This paper reports the findings of the second year of a four-year empirical research project. Its aim is to modify 'step-by-step' instructions for practical activities in a way that may enable the development of experimental design skills among junior high school learners. Each school year pupils spend six lessons doing practical activities using worksheets we provide. At the beginning of the research, the Grade 7 (12-13-year-old) pupils were divided into three groups. Group 1 (control group) followed step-by-step instructions. Group 2 followed the same instructions as Group 1, but after the experiment, they answered a series of questions on their worksheets concerned with the design of the experiment. Group 3 was required to design the experiments, guided by a similar set of questions. The impact of the intervention on pupils' experimental design skills (EDS) and disciplinary content knowledge (DCK) was measured using structured tests at the beginning of the project and at the end of both school years. Seven hundred fifty-six (756) Grade 8 pupils completed the test at the end of the second school year (April-May 2023). Over the first two years, the intervention resulted in a medium effect size positive change in the EDS of Group 3 compared to the control group (Group 1), (Cohen's d: 0.23). By the end of the second year of the project, there was only a small difference in the change in DCK between the experimental groups and the control group (Cohen's d value for Group 2: 0.10 and for Group 3: 0.12).