1. Mind the Rivers
Some worldbuilders spend years getting their plate tectonics and ocean currents just right. All of this can be fun, but you can make a convincing fantasy map without it. The thing you should really focus on is rivers. No one really cares if your ocean currents are off or your mountains aren’t in the right place. Rivers have an intuitive aspect to them that no other geological feature can match and because of this mistakes are easy to notice. To put it simply, rivers flow from higher elevations to lower ones. Most often rivers will flow into lakes, oceans, or other rivers. Rivers do not flow into a lake, wait around awhile, and then flow into a different river.
This is important because an incorrectly drawn river is jarring to look at even if you don’t know why, and because rivers are hugely important to civilization. Most major cities are built on rivers in order to take advantage of the farmland and transportation that they provide. This means that rivers can serve as a vital plot device, and as such their portrayal should not be ignored. Of course all of this can be ignored, it is your world after all, but I would advise making sure that you have a good reason for doing so.
2. Make Your Map Fit Your Story
People grow to fit their environment. You wont find horse-mounted nomads living in the mountains or naval powers in the middle of the desert. People do the best they can with the hand they are dealt. In fiction, we can decide what that best is and determine a hand that will enable it. There is of course fun to be had in drawing a map and simulating the evolution of civilization from the stone age to now, but that is not what most of us are doing. Most of us have a story we want to tell or an idea for a world we want to depict. There is no reason then to design a landscape that does anything other than to enable the story you want to tell. If you want your countries to be fighting for control of a major sea lane they there better be access to water and natural choke points. If you want your characters to venture to a distant mountain and fight a dragon then you better include a mountain or two within reach.
3. Choose a Realistic Scope
A lot of us like to think big. We draw a map of an entire world, solar system, or even galaxy and then set about filling all of it in. The problem here is that you probably wont use most of what end up writing. Trying to fill every corner of a world with detailed is fun, but ultimately futile. Above all remember that map is meant to be a reference for your players or readers, and the focus of the story is likely to only concern a small corner of the world.
Lately I have preferred to start with a single region or country that is going to be the focus of the setting. Once I have a solid idea of what that country will be I start to fill in its neighbors. Unless I have good reason to, I try to avoid writing detailed histories of these neighbors. I make a brief summary of their current state and history, and after that I try to only flesh out the aspects of this neighbor in areas where they intersect with the story I want to tell.
4. Pay Attention to Natural Barriers
Rivers aren’t the only feature on the map that shape a civilization’s development. What hinders movement in your setting can be just as important. Swamps, mountains, deserts, and seas are all important in imposing limits on an empire’s expansion, providing shelter for smaller groups, and providing places to hide all sorts of interesting dungeons.
Once barriers are in place, routes to circumvent them gain immense strategic and economic importance. Mountains, swamps, and other remote areas might also be where your world’s exiles and hermits choose to live away from society. Both of these provide opportunities for interesting conflicts or quests in a story or campaign. Characters and armies can be sent to secure and defend mountain passes, or might discover that the old hermit living in the swamp has the answer to all of their problems, but reaching him can be an entire adventure in itself.
5. Don’t Be Afraid to Change Things
Getting wrapped up in the worldbuilding is easy. It’s also easy to be disappointed. The finished product rarely resembles the initial concept and there is nothing wrong with that, except of course if you are not happy with it. If you getting into your worldbuilding and you realize that you put a mountain in the wrong place or your cities are too far apart. If you begin to get the feeling that your map is keeping you from writing the conflicts and plot lines that you want then by all means change it. Starting a new project by drawing a map can be a great way to start, but that map should not be a cage. Sometimes we need to make revisions, and it doesn’t do any good to get too attached to a map you have drawn. Keep it of course, even discarded ideas can be useful later on. Don’t be afraid to retcon the entire map if it no longer serves its intended purpose.