Test Plan

STLC (software testing life cycle)
Requirement Analysis
Test Planning
Test Case Development
Environment Setup
Test Execution
Test Cycle Closure/ Evaluating Exit Criteria and Reporting


What does a test plan include? 
(From ISTQB)

• test plan: A document describing the scope, approach, resources and schedule of intended test activities. It identifies amongst others test items, the features to be tested, the testing tasks, who will do each task, degree of tester independence, the test environment, the test design techniques and entry and exit criteria to be used, and the rationale for their choice,and any risks requiring contingency planning. It is a record of the test planning process.

What is the benefits of a test plan?
(From ReQtest)
It is the guide book for the testing process. It directs your testing approach and describes the testing practices to be followed.
It contains details of the testing scope which prevents team from putting any efforts in testing ‘Out of scope’ functionalities.
It helps to determine the time and effort required for testing the product.
It clearly defines roles and responsibilities of every team member so every individual in the testing team knows what is required of him.
It provides schedule for testing activities. Hence, it provides you a baseline schedule to control and track your team’s testing progress.
It outlines the resource requirements and equipment needs which are essential to carry out the testing process.
It can be shared with your client to give them insight about your testing process and gain their confidence.

What is a good test plan?
Simple, Clear, Easy to Read and Follow.

How to write a test plan?
First make sure what contents need to be included in the the test plan.
For example

• Introduction – what is the project all about?
• In-Scope – what will we be testing?
• Out-Of-Scope – what won’t we be testing?
• Entry Conditions – when can we start?
• Exit Conditions – when can say we’re done?
• Timescales/Schedule – how long do we think it’ll take?
• People – who’ll be working on the project?
• Test Environment – where will we best testing on?
• Risks – what could go wrong?

Then Choose a Template. There are many templates online. Compared to the old lengthy test plan, One Page Test Plan means to put or fit the most important test plan contents into one page. Therefore the readers can get a good and clear view.
Talking about "one page test plan", I found this dashboard style was really clever.

(From https://testsheepnz.blogspot.com/) 

A good test plan is very necessary and helpful.