Open up your favourite terminal (and navigate somewhere you want to download the repository to).
Make sure you have NodeJS installed. Test by entering
$ node -v
If this returns a version number, NodeJS is installed. If not, get NodeJS here.
Clone the repository and navigate to it. If you have Git installed, type
$ git clone https://github.com/NullDev/senpa-io-client.git && cd senpa-io-client
If not, download it here and extract the ZIP file.
Then navigate to the folder.
Install all dependencies by typing
$ npm install
Start it by running
$ npm start
or build it for your platform:
$ npm run build