Basically the first thing you need is Sims 4 Studio, a program that works on Mac and PC (thank goodness because I have Mac only). This allows you to open, edit, and create mod files.
Then the biggest thing is identifying already-existing interactions, traits, and other game features that are similar to the ones you want to implement and using that code to create your own mod. So like when I wanted to create a mod that makes only college students gain weight, I went to TSF’s expanding waistline mod and used a modified copy of that buff to make it happen (buffs are the little squares in the bottom left corner that affect sims’ mood, they’re used for a lot of things and can also be made invisible.)
Loots are code files that run in the background in Sims 4 to activate buffs and cause other effects, I’m not super familiar with all the ins and outs but it’s a term you’ll need to know. Zero’s tutorials (below) go into a lot more detail about these.
There are a couple of YouTube videos about modding with Sims 4 Studio that can be very helpful, especially if you’re just getting started.
One other thing I’d recommend is making sure that each individual step of a mod is working before you move on to the next one. Want to modify an item with strange properties? Make sure the model you copied is in the buy menu and can be placed in the world. Then move on to creating the custom interactions. It will save you a lot of headaches down the road if you take things step by step. It involves a lot of opening and closing TS4 and a lot of test games but it’s worth it in the long run.
Also if you’re going to be making WG mods I recommend you download MC Command Center, particularly for the weight modification possibilities. It is so much easier and faster to be able to check sims’ weight down to the thousandth of a pound with the tool rather than having to eyeball it (well, he looks a little bigger but I’m not sure it worked…). See original post for more details.
Here’s a link to the Sims 4 studio forum, it has tutorials and can give help on how to make mods (this link in particular is to the subforum where you can ask for help) Creator Help | Sims 4 Studio
Here’s a great tutorial series I found on patreon (you don’t have to be a patron to access them): https://www.patreon.com/zer0_Mods_/posts?filters[tag]=All%20Tutorials