Putting in place strong evaluation practices contributes to evidence-based decision making and helps promote a culture of continuous improvement. Ideally, you will start thinking about MEL early in your project cycle, but it is still useful to apply MEL throughout all stages of your project.
There are a range of resources available to help you implement a MEL framework that is right for your project, including different evaluation methodologies. A good place to start is Better Evaluation and Australian Evaluation Society.