First i set up my game settings, then I go through all menus for my sims/household (the sim, the mailbox, the computer etc) for all of my mod settings for the file and for each sim.
Once I have everything set the way I want - mods, aging, season length, everything - I move on!
Then I set up the calendar with holidays and stuff. I am extra with all of that lol.
I set up their home. I usually only use starter funds or they start with nothing because I like the struggle lol. I never cheat money, skills, rewards. They always start from scratch. I even have a hard time letting them keep starting funds even if it isnt a rags to riches, and I have them paying back loans w/ interest for every simoleon they started with ? choose lot traits and challenges, etc. I like the simple living one or parallel in mods where i have to either grow or buy ingredients.
All kids are born in-game. For some reason I can never start with kids. I looove family gameplay but I am just very into the whole "starting the file at the BEGINNING of their story" like i can never get comfy with starting a family that has already progressed or has money or things or skills lol. Unless it is very important to storytelling
Once my home is set up I make sure there's a few key items always:
-thermostat if they can afford it or any other heating/cooling things they might need
-umbrellas set up
-vacuum & laundry set up. I make this mandatory lol
-everything i need for my custom recipe mods is purchased and set out where it needs to be
-lighting is set the way i want it
-photos! I am a huuuge storyteller and having photos is one of my musts! I literally go into a separate file, age the family backwards, set up scenarios and everything else, just to get good family photos so it looks like theyve been around forever lol. I also get whatever i need for taking more photos as the game begins. Cameras, flashdrives (mod) and all of that.
Then I go into the world map and add/delete/change lots as needed throughout the world. Adding family members or friends too if needed. I always make sure all the places i want to go are in world and ready, and everywhere has a thermostat!
Then I press play and I immediately do whatever is needed for my mods starting out. I use snb bills to set up utilities, i open their bank accounts, i get them their jobs, i claim their beds, I get them vaccinated and go to all their checkups if needed (dr, dentist, eyedoc), get their subway passes or drivers licenses, all the things.
Then i will usually save as, and that is the start point for that particular file. And im good to begin!
Ps - i have my general starting point save file with a world set up in depth with lots i normally like to have and the calendar holidays set up the way i normally use them. So this is my starting point for new games and it is a huuuge time saver even if i change a few things. So much easier than doing this from scratch every time.
