Write a page of notes on isotopes and bring it to the lesson. Also bring the working out from the calculations that you carry out in the second video.
You need to revise what you learnt at GCSE before you learn the new material.
A guide to calculating the relative atomic mass. Pause the video when prompted to perform the calculations. Bring the calculations you do to the lesson.
If you didn't like the first video, here's an alternative one.