Instant CSS requires concrete5 version 5.6 or newer.
Once installed you can immediately start adding 'Instant CSS' blocks to any block friendly location.
Feel free to take a look at Dashboard > Instant CSS > Options so you know where it is for future reference. The default options are perfect for humans, andorians, klingons and even romulans!
<style> tags will be added automatically around your CSS.
If you ever want to temporarily disable Instant CSS from outputting code on a particular page then add the query string ?instant_css=disable onto the end of your URL.
For example, if the page you are having trouble on is located at http://forestmist.org/ you would go to http://forestmist.org/?instant_css=disable and then any output from Instant CSS blocks would be hidden.
Very useful in case styles from an Instant CSS block ever interfere with your concrete5 editor. With those styles temporarily disabled you can easily get back to editing the block that caused the issue.