To Go Anywhere, At Least Four Things Are Useful
1. 
Know Where You Are
2. Know Where You Would Like to Go
3. Have a Map
4. Have a way to Get There
Any of these four may actually be challenging to acquire.
There’s probably as many ways to do product roadmapping and define feature priorities as there are product managers. Most any skilled product manager with any degree of experience is familiar with both Waterfall and Agile methods, at least in concept. Not everyone has necessarily been formerly trained in either or both. Most often, a true pro will at least seek out self-learning resources to really understand their chosen method. They may choose to diverge from full on formal Work Breakdown Structures, (in the case of Waterful), or may not be using formerly defined Scrum methods, (in the case of Agile).
Regardless of method, in most smaller development efforts some basic ideas have to start somewhere. Whether this is with a defined Product Manager role, a product oriented CEO, the Marketing Department or wherever, you’re still at the very, very early idea stage. Long before an idea even gets to any kind of Sprint planning meeting or a line item in a Project Plan, there’s probably a high level gut check first. In smaller start-up organizations, a lot of times these are first generated in simple spreadsheets; be they Excel or increasingly something shared such as a Google Docs spreadsheet. Of course, there’s been an explosion of tools from the basic idea level through the full development life cycle, but nevertheless, there’s some very basic, simple judgments that are useful first.