The first thing you have to do, is create a workspace for Userfrosting. Press the giant "+" to get started.
Next you'll have to give it a name, and chose the "PHP, Apache & MySQL" template. Do not use the clone from git function, as it can often act weird, and not clone properly.
So, by default, C9 has PHP
v5.5.9, and Node
v4.6.1, both of which are not the best for UF.
To update PHP, follow this handy dandy Stack Overflow guide. ;)
It'll show you how to replace
To update Node, use:
nvm install 5.5.0 nvm use 5.5.0 nvm alias default v5.5.0
You'll need to install the PHP MySQL package too, for that use
sudo apt-get install php-mysql.