This is not a standard tutorial. Instead, this will cover ways that I have found to be useful in getting my first map commissions. This is certainly not unique, and I know others have found separate routes to this. I wholeheartedly encourage them to chip in with their tips too.
1. Create a portfolio
To get work you need to be able to show that you can do the work required. In fantasy cartography there are 5 types of map: indoor encounter maps (dungeons), outdoor encounter maps, town/city maps, regional maps and world maps. Ideally make sure that you have an example of each that you feel is the best that you can do. On the other hand, you can always pitch for just encounter maps, or just regional maps. It narrows your market, but it reduces the number of pieces you have to prepare.
Put the portfolio up somewhere. You can create your own website for this - or an easier solution is to use a site like DeviantArt or CGHub. I prefer CGHub becuase I find it more directly geared towards fantasy art and has a nice portfolio interface.
2. Get your work out there
This is one site for discussing maps - but most people here like making maps and aren't going to commission you. You can and should keep an eye on the mapmaking requests forum here, but also post your work elsewhere. Here are a few places that I've found it useful to create a post with my work. I update the post whenever I have something new to show. Don't post rubbish, and don't bump your thread without good reason. If you only post your best stuff then people browsing will get a steady trickle of your good work and will be constantly reminded of your name. When they need a map they'll think - ah, that guy ont he forum had some good stuff. I wonder if he's free?
Here's my list of useful forums:
You'll see some familiar faces on those forums
3. Do everything that you can get your hands on
Initially, take up everything that you can manage (and be realistic about what you can manage). Indie publishing is small and friendly word and people know each other. Make sure that you do a good job on each project, are courteous and get the work done on time. If you do this people will enjoy working with you and your name will get spread around. You'll find people start contacting you because of what someone they knew said and you'll start having to pick and choose commissions. Don't worry about the pay to start with. Your initial commissions are likely to be low paying jobs or free work. They are still worth it as they'll give you practice working to a brief. You can make mistakes and not feel too terrible about it and find your feet. As more people start asking you to do work, then you can put your rates up until you find the point where you have just enough work to keep you busy.
There will be people who want something for nothing. It's your choice whether to work for them or not. If they are private individuals then the benefits are slim. However if it will result in your work being more widely seen then it might be worth it. I was approached to do a core art pack of tiles for maptool for free. Now I've got a lot of (free) value out of the program and wanted to give something back. It also means that my art is seen by all the users of maptool - which is an audience of people who need a lot of maps. That was an easy one - I did it.
You can also send your portfolio to people. It's worth looking for small to medium indie publishers and sending an email with a link to your portfolio, or five (downsized) portfolio pieces in the email itself. For advice on this I'd recommend reading the blog of the AD for D&D:
That's how I started picking up commissions and if it works once it should work again. I'll let others chip in with their thoughts now.