I've written about my thoughts regarding product roadmaps here: https://medium.com/@jefago/how-do-you-create-a-product-roadmap-d41a42b4a543

That should cover the process fairly well. I think a quarter is a typical timeframe, although I think that sometimes a 6-week timeframe might be better (generally a bit easier to estimate what you'll get done in 6 weeks vs in 3 months). I would try not to commit to what / when features will be build during the quarter, but rather to the objectives to be achieved (e.g., what metrics to move).