How Do I make Mods?

You don't need any skills unless you want to do scripting, and even then Python is easy enough to learn with outside resources, it's just that transferring that knowledge to Sims 4 is what's hard as it's not well documented. You also don't need to do scripting majority of the time due to community tools that handle injecting for you, it's only when you require stuff that isn't covered. Almost all mods can be made in just XML (aka tuning), and is where you should focus first before venturing to scripting. Start with overrides first, things that EA made that annoy you, such as changing the time it takes to do something, anything autonomous, etc. Or update old override mods. Then when you're comfortable at least reading some of the files and understand more of what you're doing, venture into making custom files. Start small here otherwise you might overwhelm yourself. If after overrides you aren't comfortable with tuning yet, you can try Mod Constructor V4 instead to make custom files, but it's not capable of doing everything and you may still need to edit files.

Here are resources to get you properly started.
Tuning
Scarlet's Beginner tutorials (starting on page 2): www.simsvip.com/category/the-sims-4/how-to-create-a-mod/page/2/
Zer0's tutorials (for beginners and beginning intermediate, covers social interactions, buffs, and more): www.patreon.com/posts/tutorials-35031545
How to Start a Custom Tuning File: www.patreon.com/posts/how-to-start-38023976

And then you need the TDESCs and TDESC Browser as it helps you understand how files are built and what each thing does. Zer0’s tutorials linked above have 3 different tutorials on how to use the TDESC, so make sure to read them to understand how TDESCs work.
TDESCs (make sure to build your folder right, it's TDESC Folder > Subfolder for each patch TDESC): www.forums.thesims.com/en_US/discussion/976578/1-63-133-tdescs#latest
TDESC Browser (when referencing the folder in your settings do the main TDESC Folder): www.modthesims.info/download.php?&t=618199

XML Extractor if you want to update or make new mods before S4S updates, and XML File Finder (needs the extractor and in the same download) to see connections/references between each file without having to manually search for them. File Finder is very useful and highly recommended, even if you don’t use the XML Extractor for modding.
modthesims.info/d/563256/xml-extractor-for-the-sims-4.html

Tuning Error Notifier is to help you make sure you don’t make mistakes.
scumbumbomods.com/tuning-error-notifier/


There are tools that handle injecting for you for common areas, so you don’t need to focus on scripting until the time is right. This includes:
XML Injector: (interactions, loots, buffs to traits, reward traits, name objects, object relationships, states): scumbumbomods.com/xml-injector/
Injection Tools: brainblasted.itch.io/injection-tools
Andrew’s Crafting Enabler: sims4studio.com/post/72754
Mod Constructor: (not an injector, except where needed, but can be used to make interactions, buffs, traits, careers, aspirations, events, and a few others): zerbu.tumblr.com/post/181782526135/the-sims-4-mod-constructor-v4
And a couple of misc others for specific areas, like Trait Tracker Injector.

Scripting
When you are ready for Sims 4 scripting, know that there are very little tutorials and no documentation, so you have to learn through a lot of trial and error. You will need to learn Python, which is well documented and has plenty of tutorials outside of Sims 4, but it won’t help too much with Sims 4’s data.
Beginner's Guide to Python: wiki.python.org/moin/BeginnersGuide
Getting Started with Sims 4 Python Scripting: sims4studio.com/thread/15145/started-python-scripting
Tuning Inspector, which helps you figure out injection paths: modthesims.info/d/642704/tuning-inspector.html