1. Configure the repository

$ curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
$ echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list

2. Install Yarn

$ sudo apt-get update && sudo apt-get install --no-install-recommends yarn

--> Because I used NVM to install NodeJS, I Added "--no-install-recommends".

...
Recommended packages:
  nodejs
The following NEW packages will be installed:
  yarn
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 697 kB of archives.
After this operation, 4,436 kB of additional disk space will be used.
Get:1 https://dl.yarnpkg.com/debian stable/main amd64 yarn all 1.7.0-1 [697 kB]
Fetched 697 kB in 1s (1,105 kB/s)
Selecting previously unselected package yarn.
(Reading database ... 166162 files and directories currently installed.)
Preparing to unpack .../archives/yarn_1.7.0-1_all.deb ...
Unpacking yarn (1.7.0-1) ...
Setting up yarn (1.7.0-1) ...
...

3. Verified Yarn Installtation.

$ yarn -v

1.7.0