To get a silver medal for a month you need to complete approximately a full weeks worth of challenges.
For tips on completing challenges, refer to my guide for the Perfection achievement:
This achievement was bugged prior to the end of May 2017: silver medals were being counted as bronze medals. The fix was not retroactive. As such, assuming a player got the silver medal in May after the bug fix, the earliest this achievement can be unlocked is March 2018.