How do you prepare a testing plan in Agile environment?

Test plan is created for the requirements which need to be achieved.
For those requirements, which are not (yet) in the scope - no plans are needed.
Test plan is the collection, categorisation and logical structure for execution - plan - for all the test cases needed to evaluate fulfillment of those requirements, which are in the scope of the project/iteration.

The value of a test plan in agile development. Agile teams can still benefit from test plans—if those plans are limited to one page and contain only the essential information. ... The work agreement describes what the story means and how it's tested between development and QA.

I will share what we did on our org (ORACLE),

We followed scrum model of agile and worked iterations. each iteration was of 3-4 week max.

Each sprint had time for development and QA both. till the time developer used to code, QA people used to prepare their test scripts and other docs. once code is given to QA, they used to perform testing on a separete machine.

After each 3 sprints, we used to perform one integration testing. Here all the code developed so far used to get deployed on QA env and they used to peform integration testing. Even developers used to help QA people in such testing. This is kind of a model we were using in our projects and after initial hiccups it started giving results. You can reach me directly in case you need more details. wil be happy to help.

As per my experience every company has different pocess to work in Agile envinronment. Some companies do not get adopted to Agile due to less project budget and time limit. To create a test plan depends on what type of project you are working on as there are 4 main testing quadrants to create a test plan:

1) Quadrant 1 : Unit testing test plan

2) Quadrant 2 : Functional testing test plan

3) Quadrant 3: Usability, exploratory testing plan

4) Quadrant 4: Load, performance and security testing plan

So it depends on what requirements and specification you have with you and based on that you create your own test plan. Test plan should not be create only as per ieee 829 document. Go ahead and create value test plan as per your requirements.

You can use Test Link it's free and open sources.
it can help you to create test specification, test plans and test cases.