C G P Grey has made three simple, explanatory videos, which are all worth watching by electoral reformers and each one is followed by on-line debate. Although they are in an American context, the principles are universal so they should be of interest to reformers outside the USA:
The problems with FPTP: https://www.youtube.com/watch?v=s7tWHJfhiyo
Gerrymandering explained: https://www.youtube.com/watch?v=Mky11UJb9AY
STV explained: https://www.youtube.com/watch?v=l8XOZJkozfI. Although this oversimplifies the calculation of the surplus, it is still worth watching.
Non-reformers should watch them even more than reformers should. The videos would be educational for them! But non-reformers probably won’t see them unless you advise them to watch.