Involve Testers from the Beginning

Testers need to be involved from the beginning of a project's life cycle so they can understand exactly what they are testing and can work with other stakeholders to create testable requirements.
Defect prevention is the use of techniques and processes that can help detect and avoid errors before they propagate to later development phases. Defect prevention is most effective during the requirements phase, when the impact of a change required to fix a defect is low: The only modifications will be to requirements documentation and possibly to the testing plan, also being developed during this phase. If testers (along with other stakeholders) are involved from the beginning of the development life cycle, they can help recognize, discrepancies, ambiguities, and other problems that may affect the project requirements Testability, correctness, and other qualities.

A requirement can be considered testable if it is possible to design a procedure in which the functionality being tested can be executed, the expected output is known, and the output can be programmatically or visually verified. Testers need a solid understanding of the product so they can devise better and more complete test plans, designs, procedures, and cases.
Early test-team involvement can eliminate confusion about functional behavior later in the project life cycle. In addition, early involvement allows the test team to learn over time which aspects of the application are the most critical to the end user and which are the highest-risk elements. This knowledge enables testers to focus on the most important parts of the application first, avoiding over-testing rarely used areas and under-testing the more important ones.

I have taken this topic from the book” Effective Software testing” by Elfriede Dustin.