There are two methods to backup your save games. Most of the time it’s best to use method 1. Method 2 is best to be used if you are playing the game and want to backup your save game before a big event in your game (like moving) and you want to be sure nothing goes wrong.
Save games can be found in this folder: Documents\Electronic Arts\The Sims 4\Saves
To backup your save games, all you have to do is copy the file Slot_<Number>.save and maybe Slot_<Number>.save.ver<Number> to a save place (external disc, flash drive, other place on your hard disc, DVD, …). You could create a folder SimsSaves for this.
Hover over the little folder icon to find out which save is which file.
Since the save games do not have names that tell you what is what, it is best to create a folder for each of your games and put the current save game and it’s backups in there. Use a name you can remember for the name of the folder. I do not recommend renaming the files itself since you would need to remember to name them back.
You can also use the save as option in game to give the game a new name. This way you don’t overwrite the old one. The save game gets a new slot number.