Find your niche. If your focus is on health, is your larger focus on men's or women's health? Older or younger readers? A specific disease or condition (cancer, lupus, mental illness)? Narrow your focus to better understand your editorial strategy.
Create a business plan around your proposed magazine. A business plan (templates are available for free from SCORE.org) will help you build a mission and vision statement, further narrow your focus, develop competitive intelligence (know what the competition is doing), and get you thinking about funding. Business plans can be one page to 100 pages long.
Build an editorial calendar. Editors and journalists use an editorial calendar to guide their content through the year, so themes remain consistent and stories can be developed months in advance. Stories may focus on an annual event or trade show (breast cancer awareness month, for example), seasons, holidays and more. Determine what "events" occur in your niche and build your editorial calendar around those moments.
Assign stories. Whether you are the only journalist or you have several writers or freelancers on board, start executing against your editorial calendar. Plan to write and design three months out; if it's January, you should be working on your March or April issue.
Design your first issue! Create a logo, conceive your cover page, build your editorial well. Your editorial well is the way content flows through your magazine, traditionally with an editorial up front, news briefs, a feature well, shorter feature items and a catchy last page feature.
Get funding. Perhaps the hardest part, unless you are able to fund your magazine personally. Use your business plan and your first issue (designed in mock-up with stories placed in the magazine) to gain advertising or secure investors or both.
Publish your magazine. How you distribute your magazine (subscriptions, magazine rack, giveaway) may determine the number of copies you wish to publish. Your printer may also dictate that number, based on minimum order number.
Distribute your magazine. Give away many of your first issues to gain readership. If you have advertising support, you might be able to distribute your magazine for free. Otherwise, you'll need to sell subscriptions and build a readership base that will support the cost of publishing your magazine.