<p>Like, testing for : </p>

<ul>
<li>invalid inputs (strings containing "'"s)</li>
<li>giving random id's in url parameters to access "unauthorized" pages</li>
<li>prevent sql injection</li>
<li>...</li>
</ul>

I would recommend reading this brilliant
book
to learn more about the test plans.
In particular a test plan is much more than a list of techniques like you mentioned (these should most likely go into the details for the security testing)
It should at least contain:
The answers to the main questions:
why bother (what is gained by testing)
who cares (who are we working for)
how much (..testing will be done)
You would not go wrong providing also:
Platforms
Methods (positive testing, load testing etc)
Functionality you're planning to test
Acceptance criteria
And there is plenty more things to talk about in the plan.