Choose the motivation for your character. What goal does your character need to accomplish by the end of the novel? This can be as simple as getting a new red wagon for a child or as complex as getting over the death of a spouse for your flawed hero. All of your characters should have an underlying motivation.
Begin writing. This is only for your reference, so don't worry about spelling or being grammatically perfect. Write about your character's childhood, family, broken hearts and greatest triumphs. As you write, you should get a feel for who your character is. Draw from your own life experiences as you construct your character's past.
Give your character some memorable quirks. A strong Boston accent allows your readers to hear your character's voice as they read. Perhaps your leading character's best friend is her English bull dog.
Describe your character's physical appearance. Be as detailed as possible. Again, you might not use all of this material once you begin writing your novel, but you'll find these details effortlessly leaking into your story. Reference the character's back story. For example, your character could walk with a slight limp as a result of an old hockey injury.
Name your character. This step is last because most people have associations with names, and you want to avoid developing your character based on your personal associations. If you're struggling, look through a baby names book. Look for names that have meanings similar to your character's traits.