Write out a list of the topics or areas of research you need to investigate in order to make your fiction come alive with believability. List items can be as specific as the kind of sword carried by ancient Roman footsoldiers in 200 AD or as general as social attitudes toward those with leprosy in medieval Italy.
Look for information at your local library or on the Internet about that topic. If you are writing about ancient Rome, read books, watch movies and engross yourself in everything from prices of cloth to Colosseum battles. If you're writing about a jewelry thief, find old court cases and interviews with convicted — and, if possible, acquitted — criminals. Find out what they did, why and how. If your jewelry thief will be robbing a museum, research museum security procedures.
Plan out your characters. Three-dimensional, engaging characters are a keystone to nearly every novel. Try to create characters that are extremely detailed. You should know their favorite color, their dreams, their nightmares, that boring job they had when they were 15. Write down as many details as you can think of. Not all of this information will be used in your novel, but you should know it in order to fully develop your characters.
Research your characters' professions. If they are specialist doctors or astronauts, try to arrange an interview. Offer to take your interviewee to lunch if possible, as recompense for their time and expertise. Then go back to the books and Internet and read more information about the profession. Anyone you interview who makes a significant contribution to your research should be noted in the acknowledgements of your published novel.
Study the behaviour of people who are not the same sex or gender as you. Try to ascertain why they do the things they do. When they make mistakes or when they are successful, ask yourself how they did it and how might you or other people of your gender do the same. Write down all your findings and use this information in your writing.
Write the novel. If you find yourself staring at a blank computer screen, then chances are you need to go back and do some more research. Research provides ideas and can open up passageways in your story that you may not have realized were available.
Rewrite the novel for spelling, punctuation, grammar and sentence structure. Fact check as much as possible and keep a file or notes on where you got the information you used in the novel. If your readers feel that you didn't do your homework, they will not be engrossed in the story.