Instant JavaScript requires concrete5 version 5.6 or newer.
Once installed you can immediately start adding 'Instant JavaScript' blocks to any block friendly location.
Feel free to take a look at Dashboard > Instant JavaScript > Options so you know where it is for future reference. The default options are perfect for humans, robots, mechanoids and other sentient life forms.
<script> tags will be added automatically around your code.
Ok since you asked... yes you can specify your own <script> tags if you wish. As long as your JavaScript starts with <script... then your code will not be automatically wrapped in HTML5 style <script> tags.
If you ever want to temporarily disable Instant JavaScript from outputting code on a particular page then add the query string ?instant_javascript=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_javascript=disable and then any output from Instant JavaScript blocks would be hidden.
Very useful in case you ever run into a JS error that interferes with your concrete5 admin bar.