What is a plan statement/how do I write one?
I'm not familiar with the phrase, but I am guessing your assignment is to create a plan or procedure for some task to be accomplished. Your plan statement, then, could be considered similar to the thesis statement of a piece of writing.
As such, the plan statement would be the introductory summary or preview of the rest of your paper. It would state exactly what it was that you intended to accomplish through the carrying out of your plan and might include the reasons why you needed to do this.
The remainder of your paper would provide detailed information about your plan, covering such concerns as equipment you may need, assistance you might require, the timeline you envision for completing the task, and the hoped-for outcome when the work is finished.