If you installed Homebrew in the previous step, use it to install NodeJS:
brew install node
If not, you can click the big, green INSTALL button on the NodeJS web site.
Node version numbers are confusing, but you should using either the LTS version (4.4.5 as of this writing) or the Current version (6.2.1).
After Node is installed, test your installation by running the following command:
npm install -g bower
You should see an animated cursor for a moment or two, then something like the following:
/usr/local/bin/bower -> /usr/local/lib/node_modules/bower/bin/bower
/usr/local/lib
└── bower@1.7.9
If the npm command isn’t available, open a new tab in Terminal or iTerm2 and try again. Opening a new tab should make node and other related commands available to you from any directory, if they weren’t already.