Set aside a portion of your house to serve as a home office. This is where you should do most of your at-home writing. Make sure it's free of distractions like TVs or video games and that any books nearby are those which solely pertain to writing. A big part of writing at home is the ability to keep at it without becoming distracted. A set place to work goes a long way towards establishing that mindset.
Crunch your monthly budget to figure out how much you hope to make as an at-home writer and how much time you need to invest to make it. If you don't have any steady gigs lined up, it's a good idea to start with at-home writing as pure gravy. Keep your day job to pay the bills and work on the freelance stuff as a means of augmenting your salary. When work starts to pick up, you can adjust your hours accordingly, spending more and more time on your at-home writing.
Print up clips of previous writing features, or save them as electronic files and prepare them as part of your portfolio. Editors will want to see samples of previous work before they hire you, and you want to be ready with your best when they do. If you don't have previous writing samples, start a blog or create articles for eHow. They provide good experience and can help demonstrate your capabilities.
Cultivate writing opportunities through websites like Writerlance, Elance and Get A Freelancer. Check Craigslist for writing options and purchase a copy of "The Writer's Market" for perusal. Not only will these resources help you find writing jobs, but they can offer advice on polishing your prose and marketing yourself.
Send out query letters to editors listed on freelancer's websites. Include samples of your work and ask them about writing opportunities for their organization. If you send the queries out through snail mail, include a self-addressed stamped envelope so they can reply to you easily. Include your email address and phone number on any piece of correspondence.
Get clear instructions about deadlines and pay rates when you're given an assignment. Write quality work, stick to your schedule and meet your deadlines come hell or high water. Pay attention to any suggestions or guidelines your editors offer and adhere to their admonitions. If you are reliable, you'll begin to build a reputation, and editors will approach you more and more for additional assignments. As they do, continue to build your workload until--ideally--you have enough steady income to stay at home and write full-time.