brew install yarn on macOS or Linux.
choco install yarn on Windows.
With those external installations, you have yarn ready and functioning without interfering your current node installation - except you've installed node directly via brew. Also no need to migrate globals when upgrading node via any node version manager.
Then the rest works just like it does with npm.
mkdir supercoolsomething
cd supercoolsomething
git init
git add remote origin something
yarn init
yarn add react@15.4.0-rc.4
yarn add babel@latest
yarn add runjs
The generated package.json looks familiar, just like we know it from npm.
The installation of packages is fast and produce the same node_modules structure on every other machine - except on different operating systems.
What I noticed is, that yarn accesses its registry at registry.yarnpkg.con. You may need to add an exception to your firewall or proxy.
It's ready for production now; because yarn isn't bloody fresh code just released a week ago or so. yarn is the product of a corporation between Facebook, Exponent, Google, and Tilde.