Identify the demographic that you're marketing to. If you're looking to guide a family of various fitness levels on day hikes, it's a lot different than "extreme" technical climbing.
Once you've done that, identify the regional market where people of that demographic have some disposable income (aka "rich people"). If folks are struggling to pay their bills, they are not paying for outdoor guides. Urban dwellers are often looking to get some "nature time" in. Maybe partner with a local lodge or hotel to offer accomodations as part of a package. From community pin up boards to paid advertising, to internet ads, the options are endless. Get interviewed on podcasts. Offer free seminars to corporate organizations regarding the health benefits of being outdoors. REI put together a beautiful booklet on this subject.
Next figure out how to sell to those folks. Identify their needs and market the services to meet those needs. What are they getting for the price of your service? Would you pay for it? Always keep the customers' perspective on your mind. What can you offer that will make them part with their prescious money? Getting your first few clients and working out the unforeseen issues will be initially challenging, sometimes discouraging, and will require commitment to the long-term goal. Once you've got some clients, word of mouth is your best friend.
As with any business, you must continually educate yourself. NOLS and other schools offer plenty of books and classes. Don't forget a business license, liability insurance, first aid training, and necessary permits if guiding on public land.