Prerequisites:
Open the plugin. A button displaying a red circle must be shown. If it's pressed, it means you can start your scenario. The plugin is recording everything you do within this Firefox instance.
Do whatever you want to be recorded.
In the end, save your scenario; you will notice that Selenium IDE's scenarios are html files.
You can also open files from other users. For instance, copy and paste the code below in a new html file and import it via your plugin. You will be able to run the -very simple- scenario.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head profile="http://selenium-ide.openqa.org/profiles/test-case">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<link rel="selenium.base" href="https://www.google.com/" />
<title>sample-test</title>
</head>
<body>
<table cellpadding="1" cellspacing="1" border="1">
<thead>
<tr><td rowspan="1" colspan="3">sample-test</td></tr>
</thead><tbody>
<tr>
<td>open</td>
<td>/</td>
<td></td>
</tr>
<tr>
<td>type</td>
<td>id=lst-ib</td>
<td>Wikipedia</td>
</tr>
</tbody></table>
</body>
</html>
This DSL (domain specific language) is commonly named "selenese".
Its most common functions are listed here.