Might be a dumb question, but how do I backup?

I play Stardew on a PC, windows 10. I'd like to try a couple of mods that change a lot things in the game, but I'd like to know how to backup the entire game. I was lucky I didn't loose everything with my latest fooling around. I tried backing up just the Stardew Valley file from the Program File/Steam/steamapps/common/Stardew Valley. It looked to me like that would have all the info I needed to return the game to a previous save. I downloaded and installed a mod, but the mod wouldn't work. When I tried to reset my game with the saved file somethings returned to the older save, but other things didn't. I was lucky that enough returned that the game worked okay for me. I know just enough about working with computers to be dangerous.

Can someone tell me how to do a complete backup the right way?

 

See saves on the wiki for help manually backing up your save file. If you have SMAPI installed, it will make a daily backup of all your saves too.

(I don't recommend using XNB file replacement mods, those often cause issues. SMAPI and Content Patcher mods are safer.)