Step by Step Guide:
(for those like me who needed more info the first time)
4.) Arrow down until you see this:
5.) Paste "
- git clone https://github.com/discourse/discourse-spoiler-alert.git" beneath that so that all together it looks like this:
6.) control + O (to save
app.yml), hit enter
8.) control + X (to exit app.yml), then hit enter
git pull (this updates stuff)
./launcher rebuild app (restarts Discourse and includes the plugin)
^ @AdamCapriola showed me that stuff and I did exactly the way I ordered it and it worked for me.
It may seem like an obvious order to those more familiar with it, but that's what I needed to do it successfully the first time. Now I can do it easy peasy.