Is CSLoL Custom Skin & Sound Manager free?

Yes. CSLoL Custom Skin & Sound Manager is released under GPL-3.0 and costs nothing. Neither it nor League of Legends has any paid content.

What do I need to run CSLoL Custom Skin & Sound Manager?

League of Legends, which is itself free. CSLoL Custom Skin & Sound Manager is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from League of Legends's mods list. You can also install it by hand — full steps are on this page.

Roughly 45 MB, on top of the base League of Legends installation.

Yes. The source is published at github.com/League-Custom-Skin/CS-LoL-Manager under GPL-3.0.

CSLoL Custom Skin & Sound Manager — Free League of Legends Mod · PlayBound