Nim games are very famous in Competitive programming. Generally, I try to guess any random idea, sometimes it works and most of the time it doesn't :(

So, I am wondering if there is any general approach for these problems because I am getting terrified of games.

These days there are tons of problems on string transformation. I don't know any particular way to solve them... sometimes guessing works but surely that's not a good way. So I was wondering if there is any general method to attempt such problems, please share.