Go to the official WickedWhims website (search "WickedWhims Sims 4"). Click the download button. It is 100% free, though you can donate.

The keyword includes "free" because many custom animation creators for The Sims 4 lock their best content behind Patreon paywalls (often $5–$15/month).

However, the core frameworks are absolutely free:

Beware of scam sites. If a website claims to offer a "Child Woohoo Mod .exe file" or asks for your credit card for a "free download," leave immediately. Legitimate Sims 4 mods come as .package and .ts4script files, never .exe.

Disclaimer: This article discusses mods that alter the intended age ratings and core mechanics of The Sims 4. Please check your local laws and EA’s Terms of Service before installing. This content is for educational purposes regarding mod functionality and community trends.