Provide suggestions, not just objections.
If you disagree with your manager about something, it's important to stay calm. You can disagree constructively by showing respect for her point of view and demonstrating that you care about achieving the best result for the organization. You want to show that you're trying to collaborate, so link your idea directly with your manager’s goals and concerns.
Remember to provide suggestions your manager can act on; don't just list of directions. Be specific about how your idea can prevent pitfalls, and present supporting data to show that your proposal is fact-based rather than emotional. And always try to give a range of options. Suggesting different possibilities signals your flexibility and invites your manager to be flexible too.