Before you create milestones you need to do the following:
1. Conduct a survey of your user base or customers and be able to validate what you are building
2. Once you do the research it will help shape what the customer wants
3. Based on what the customers wants map out what are the features then prioritize based on releases.
4. Based on the releases you can speak to investors about what their capital will be used for and what you plan on achieving.
5. Obviously you will change course but investors want to know you have thought through it.
6. Communication is key once you have investors.