Is HoloCure QoL Toolkit free?

Yes. HoloCure QoL Toolkit is released under Community / External hub and costs nothing. Neither it nor HoloCure - Save the Fans! has any paid content.

What do I need to run HoloCure QoL Toolkit?

HoloCure - Save the Fans!, which is itself free. HoloCure QoL Toolkit is an add-on and will not run standalone.

How do I install HoloCure QoL Toolkit?

The PlayBound Launcher installs it in one click from HoloCure - Save the Fans!'s mods list. You can also install it by hand — full steps are on this page.