Once you download the files, you'll need to unzip the folder (my computer does it automatically). Then, find your mod folder: Documents/Electronic Arts/The Sims 4/Mods.
Once you've done that, simply place all the files ending in .package into your mods folder.
You can organise them in subfolders if you'd like, the game will still read them fine.
Finally, once you've loaded the game, make sure you enable custom content and mods in the options menu.
In the "Other" tab, make sure you've ticked these two options circled below:
If the mods/CC are installed correctly, you should be able to see a list of them in 'View custom content'
Now you're all good to go! It's that easy!
Keep in mind that mods and custom content aren't screened by EA, and they can crash your game. If you decide you want to uninstall the mods or custom content, just take them out of your mods folder, and delete them from your computer, or save them elsewhere.