A nursing dx is a clinical judgement about patient responses to actual or potential health problems. It provides the basis for selection of nursing interventions to achieve outcomes for which the nurse is accountable (Nursing Dx Handbook). The process is the assess, dx, plan, implement, and evaluate. When assessing the client you get the baseline info needed to start the nusring dx. Like a patient with COPD (med. dx) would have a nursing dx of Activity Intolerance related to dyspnea. You are focusing on the med. dx but making a nursing dx based on your findings and then figuring out ways to help the patient successfully. i don't know if tis made sense or helps but there are books aviailable to help. It was confusing to me at first too.
Mosby has a book called: Nursing Diagnosis Handbook 7th ed. by Betty J. Ackley and Gail B. Ladwig.
Good Luck!

One of the best ways I've ever heard things put is that the nursing process is a systematic problem-solving method. The process of Assess - Diagnose - Plan - Implement - Evaluate can be used for any patient problem, and in fact, has broader application for every problem, when you break it down. Assess: you gather all the data and cluster it into related catagories to lead you towards a meaningful - Diagnosis: Statment of the problem in such a way that a specific understanding of the issue is addressed and can be acted upon which you then create a - Plan: to address the problem understanding the reasoning behind each action you are going to take and with a specific goal in mind so that when you - Implement: taking measurable steps on the plan and observe and document your results against standards you (or others) have set you can then - Evaluate: the effectiveness of your actions and revise if necessary. Technically, organized thinkers have probably been doing this instinctively every time they solve a problem - they just haven't put labels on the steps of the process. The other really crucial thing to remember when differentiating a nursing diagnosis from a medical diagnosis is that medical diagnoses can be good starting points for nursing diagnoses, but they are quite limiting in scope - nurses look at whole patients, while the medical model looks at disease processes only. Or to put it another way - doctors treat diseases - nurses treat PEOPLE. A nursing diagnosis can deal with parenting, spirituality, energy, coping, body image - these are things that a medical diagnosis doesn't touch or even hint at most of the time. Don't let the medical model trap your thinking. Remember that our profession is designed to assist clients to return to optimal levels of wellness - and that wellness is individual to each client. So a good question to ask yourself when formulating a nursing diagnosis is "What is baseline for this client, and where are they RIGHT NOW that is different from that?"

And of course, as always, take what's useful that you can from those in the profession and fellow students that helps you, and toss the rest. Hope something here was useful.