Decide just what "epic" means to you. You may decide that the world you create will primarily be epic in terms of its physical size, length of history, culture-related details or a combination of all of these, but you should decide which is most important to you as a creator and devote the most time to this aspect as you create.
Determine your universe's geography. Geography is often a good place to start because it will be easier, later, to figure out certain details about your people and cultures if you already know how factors such as environmental topography and diplomatic relationships among neighboring groups affect them. Sketch out rough maps or solar systems to help you pin down details.
Populate your universe. Once you've determined physical environments, fill them with plants, animals and people. For the most realistic environments, think in terms of creating the most complete ecosystems possible by deciding where various creatures exist in relation to one another in the food chain and thinking about how the natural environmental factors (such as humidity, temperature, elevation and weather) affect the wildlife.
Create cultures for your populations of intelligent life. For each type of people you want to have, make decisions about their ways and traditions regarding daily life/survival, food, clothing, shelter, language (for a truly epic universe, consider creating entire languages), spirituality, marriage and family, political structures and war and fighting practices.
Write a history for the world (or worlds) you've created. This history will largely consist of the histories of different cultures of people, but it should also include information about how various cultures have interacted with and affected one another. You should also make decisions about how the various peoples keep track of their history (whether they have reliable records or rely on myth and oral history) and how accurate their understandings of their own pasts are.