Making the user pay attention to what’s on the screen instead of having him press next next next

I work on a fitness and recovery app which has an on-boarding process which requires the user to position 2 sensors on his body and pay attention to some small tasks on the screen (like put the sensors on your body now and press "I am ready" when done).

In turn, the users mindlessly press next without reading. The screens look like this

There's a small instruction and a video displaying how to position the sensor or strand the strap. We put emphasis on the large video as a large part of our user base are elders - videos works better than reading when they have eye problems.

As most of the tasks in a screen take at least 15 seconds to complete, would displaying a popup with a message (We don't think you properly read what you have to do) if the user pressed next under 10-15 seconds?

To not annoy the user in the long run, we can only show the popup only the first 3 times the user goes through the flow, as after that he will get used to the steps and may prepare in advance for them and be faster.

Are there other known solutions to dealing with lack of attention in an on-boarding process?