Manually :

1) Get the files

Clone the repo with git (install it if needed) :

git clone https://git.popcorntime.io/popcorntime/desktop.git

2) Install node.js

sudo add-apt-repository ppa:chris-lea/node.js

sudo apt-get update

sudo apt-get install nodejs

3) Install node.js requiered dependencies

sudo npm install -g grunt-cli bower

4) enter the directory

cd desktop

5) Install Popcorn-Time dependencies

npm install

6) Build with grunt

grunt build

7) Get the built executable

You'll find the executable inside the build/releases folder

7bis) Start the live-development

grunt start