Frankly, if you've never done anything like this before it may be a bit much on your first go.
That said, the manual is straight forward and you could manage it with some bit of extra googling. I can't explain it to you step by step, because it'll take too long and it's already explained in the manual under Getting Started: Local, I'll just try to point you in the right direction.
First, you need OS X. If you don't have a OS X or a linux machine it won't work.
Second, you need to download the zip and unzip it (the location doesn't really matter)
Third, read the manual how to install it:
Installing dependencies
To install dependencies:
brew install couchdb
npm install
What does that mean? Well you need couchdb and npm. What are those, and what's that brew?
Homebrew or brew for short is a package manager for os x. What's a package manager? A package manager or package management system is a collection of software tools that automates the process of installing, upgrading, configuring, and removing software packages for a computer's operating system in a consistent manner.
You get it by running this in the console:
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
The script explains what it will do and then pauses before it does it.
After the installation you can run that first command it mentioned before:
brew install couchdb
And it should install couchdb.
Then it says you need to do npm install. Well what's that? NPM is also a package manager. It's a package manager for Node. Here's a tutorial how to install it: http://coolestguidesontheplanet.com/installing-node-js-on-osx-10-10-yosemite/
You must also have the Google Cast Chrome extension, as well as a Chromecast setup application.
You'll also need at least one Chromecast :)
After that there are some steps to register and set up your chromecasts, then we get to:
Application Config:
Still in terminal, go to the unzipped folder location, like ~/greenscreen-master/
now you can do:
npm run create-config
then in ~/greenscreen-master/public/js/gscreen-config.js edit the chromecastApplicationId field to your Chromecast Application ID.
You can configure the couchDB and sever details in config.json (which the npm run create-config command created).
Now you're on to
Starting the Server
I think it's written well enough that you should get by, if you really wanted to... Like i said, it may be a bit much for your first time. If you've got any question just ask and i'll try to answer them. :)