WifeyClarky
rosemow has covered all the basics on creating a master game template. There is one more step you can take that will keep you from messing it up by forgetting to do the save as immediately on opening the save and then playing or when you amass a lot of saves or inadvertently overwriting later on.
Before you start the game to begin making the master, rename the saves folder in the "The Sims 4" or move it elsewhere if you want to play saves you've already made in it again sometime or just delete it if you want to. When you start the game next, a new empty saves folder will get created. Then you create a new game, make and place the mandatory sim. Then you can go ahead and make your master template by installing the lots and sims you want from your Library. Once done, do a save as and name it whatever you want to call it going forward. Then quit after saving and copy the new saves folder to somewhere safe where you can find it easily later. Then you can restart the game and play on in that saves folder as long as you like using and renaming the template each time you start a new game by using save as.
When it becomes time to use the template again, like when you get a ton of saves in the folder, you can just move the current saves folder out and copy the saves folder with the master template back over to your "The Sims 4" folder and start a new bunch of save games using your master template. Doing that helps to keep the saves folder small at any given time and makes it a little faster loading the panel when you use the Load option. And the biggest plus is that you'll always have a backup safely stored separately from the game files.
Hope this helps.