The product development process is the instrument every product manager should use. It is refined over the years by the product manager as he works on more products. The product manager could start with an approach like this.
An approach to implementing a product development process:
1. Conceive: identify the focus areas for the product
2. Plan: market research on ideas, business case, customer interviews, roadmap
3. Develop: timelines, features, specs, user stories
4. Iterate: test assumptions, do alpha, beta, etc. until the product is ready to ship / product-market fit
5. Launch: work with marketing, sales, legal
6. Steady-state: collect metrics on how the product is used
7. Maintain or kill