The first step in writing an epic novel is determining that you need the length of an epic novel to tell your story. Most epic novels are long for one of two reasons: either the story takes place over a broad length of time or the story requires multiple characters whose story each must be told. If you can cut your novel down and still tell the story you are aching to tell, now is the time to do it: before you begin writing. If you absolutely cannot imagine telling your story in fewer than 600 pages, then admit it: You are about to write an epic.
A good way to begin the process of writing an epic novel is to boil down your themes to a few simple sentences. Write these themes down and keep them in a visible place at all times. This will allow you to focus on the important message your novel will tell the reader. Because the act of writing an epic will require a tremendous expenditure of energy, it makes little sense to spend that time writing a novel that does not have something significant to say.
Many writers cannot exist without an outline to guide them. Some writers, such as James Michener, produce outlines for their epic novels that are as long as the actual novels of other writers. Some writers just like to wing it, letting their imagination lead them on the journey toward completion. In the case of an epic novel, however, it always behooves a writer to produce an outline. Your outline does not have to be as dense as Michener's, but it is vital to know in as much detail as possible where you are at any time. Why? Because one day you may be slugging through Chapter 10 and the next day you may feel like an unseen muse is guiding you to magic in Chapter 47. An outline will help you keep track of the overall story. This is the place where an assistant is highly preferable, though certainly not a necessity.
Once you begin writing, the most important thing is to set a goal. Unless you are Stephen King, capable of producing two or three novels running more than 500 pages in a single year, you can expect to be working on your own epic novel for half a year to several years. Let's say your epic novel is about 750 pages. That is roughly 225,000 words. If you write every day for a year, that means you will be averaging about 600 words a day. That's two pages, more or less. When you think of producing just two pages, or 600 words, every day, the task of writing an epic novel may seem less daunting. The key to finishing your epic novel is to think of it not in terms of writing another 150,000 words or another 300 pages, but just another 600 words tomorrow. And the next day.
Finally, unless you are already a full-time writer or someone with the ability to spend several hours a day at the computer, you will probably be spending the bulk of your day doing something besides writing. Just because you are not writing does not mean you won't be thinking about that novel in progress. Carry a notebook or a digital voice recorder with you. Any time you get inspiration, from coming up with a perfect name for a character to a major plot development you never thought of before, record it. You may think it's such a great idea that there's no way you'll ever forget it, but don't be fooled. You might forget either the entire idea or the nuances that make it memorable, so write every idea down, even if you can't yet figure out how to use it.