I always start with a map. Either one that I've made or a compelling one I've found. In fact, if I'm ever stumped on where to go next in a game -- regardless of genre -- I start looking at maps.
In the old days, that meant flipping through game modules, encyclopedias, National Geographic, and even my dad's glove compartment. These days, we have an embarrassment of mapping riches. So many amazing artists out there have crafted maps for everything from galaxies down to cabins in the woods. So, how to consume all those riches in order to build a compelling campaign map and gather a bunch of other maps for impromptu encounters, lairs, and sites? One bite at a time.
1.1 Build a Very Small World
Your starting area map is your first problem... The temptation is to make a giant map that stretches across the continent or the world. Calm down. Start very small -- I usually do a 10 x 10 hex map or the equivalent size if I'm not using hexes. 60 x 60 miles is a *lot* of space. And as you'll see when we start stocking these things, there is a lot of adventure to be had within every 6 mile hex.
Freeform this as much as you want within the genre and setting you're going for. You want interesting places for your player characters to explore, but you don't want to be so detailed that it takes forever to develop an area.
It's also important to think about barriers and bypasses here. I don't want to say you intentionally want to slow your characters down, but if you think about the movement rates of the game system you're running, as well as the resources available to a starting and developing character party -- you want to make sure they don't overrun your starting area.
In the map below, my character party is hemmed in a bit by the mountains to the north, as well as the hills and marsh/swamps to the east. But, there are some built in bypasses, as the characters' skills and resources increase.
With access to boats, the characters can move quickly through the swamp. The mountains have passes that can be discovered. When the party gets access to horses, they can easily outdistance my map over the grasslands to the southwest. I'll have to use sites, encounters and hazards to slow them up that way. And expand my map as the characters' abilities and desire to explore increase.
The important thing to remember about your starting map: Start Small. Grow with the Game.
1.2 Lay Down the Zones
Your very small world should comprise a number of zones -- areas where there may be similar landscape. This helps you customize how the game runs in that area -- more on that later -- but also simplifies how you run sessions. Everything springs from the map -- movement rates, hazards, monsters, sites and even weather, if you go that far. It's fine to spend some time here -- but don't obsess too much. You're still broad strokes. I made the map below with worldographer in about 20 minutes.
I guarantee that I will go back and tweak this map a few times before I'm ready to run it (I hate the way the river looks, for instance). Aside from that, this is going to be the very small world where the village of Bexley sits (I'm cheating a little bit -- that's step 4). Characters will fan out from here looking for gold and glory.
You can tell by looking at the map that this is going to be a fairly traditional "Northern European" style fantasy setting. I've got distinct zones. I've got barriers and bypasses. Now, I can easily clarify movement rules based on these areas, as well as build encounter tables, place a few adventure sites and lairs, and generally think about and get inspired by the map itself.
A note about scale: I use 6 mile hexes for overland maps -- about 10 km. Some folks use 5 miles. The key here is to use a scale that contains a good amount of space, but that isn't too big. A 24-mile hex, for instance, is gigantic. It's almost 500 square miles. A 6-mile hex is somewhat over 30 square miles -- a manageable size. In my own campaign, there are 0-3 sites or lairs in a 6 mile hex. This gives players plenty to discover in a given hex and plenty to jenga when they find it. Your kilometers may vary.
1.3 Find a Bunch of Other Maps
Now you need maps. They don't have to follow an artistic theme or even be that detailed -- these are for inspiration and impromptu encounters, lairs and adventure sites. Here again, you can spend way too much time in "research mode." Go search for fantasy adventure maps and grab the first 10-15 that appeal to you. Hell, you probably already have several hundred on your hard drive right now. Pick a few and move them to a relevant folder for later use. I usually like to give the image files descriptive, but generic names and keep them all in the same folder -- so for instance: dungeon1, dungeon2, lair1, lair2, etc. This way, everything is there, I don't have to try and remember what type of map it is, and I can just grab one as an emergent encounter happens.
A point about stealing: I use the word "steal" a lot, but I don't mean actual larceny. I just mean "get someone else to do your work." Don't steal from our RPG creators. These folks spend a lot of time and effort to learn and perfect their craft, and then a lot more time and effort to self-publish their creations. I've done this and I can tell you: It's a lot of damn work. One of the best ways to grab a bunch of maps -- especially if you're obsessive about the art style being consistent -- is to join patreon and become a patron to several map artists. You can also find these creators in the roll20 Marketplace, DrivethruRPG, and even a brave few running their own websites and selling their art there. You can also find a ton of RPG map art out there for free. Check out my sandbox resources (pending) page for artists that I work with both for personal and professional projects.
1.4 Tools You Can Use
I've mentioned worldographer in 2 of my 2 posts so far. I can only tell you what works for me; this tool is an absolute godsend. There are others -- a lot of new web-based tools -- but for the money, worldographer is my go to for area mapping. The output is crisp and clean, you have some interesting artistic/stylistic options, and your map is easily modified if you need to make changes. The output resolution is as crisp as you want it to be -- especially if you're using the map in a VTT setting. And you can very quickly build an area map, then drill down to even more detail if you're so inclined.
I use Inkscape (and I shouldn't...this is my obsession) for "close area" maps. Think the Keep on the Borderlands area map or Horror on the Hill. I have a few tools and techniques that work for me, and (if I control myself) I can draw a relatively interesting map of a close area in an hour or so. I may actually do a tutorial, because I really like Inkscape for this task. I'm sure for-pay tools from Adobe and others are more feature rich, but Inkscape is free and it has always worked very well for me.
I use GIMP for dungeon maps -- whether I'm drawing them from scratch or modifying a map that someone else has created. Another free tool -- comparable to Photoshop -- GIMP allows me to get as detailed as I want to get and keep everything in layers. Probably another possible tutorial. I really like to make dungeons with simple (even boring) styles -- this cuts down on the development time considerably. Spend your development on what's in the juicy center of the dungeon. Not on the décor. Great thing about GIMP and similar apps is that you can build everything into layers -- so I do the dungeon outline on one layer, visible features in one layer, secret features in one layer, numbers/notes on one layer, etc. Then, you can hide hidden things and export a map for your VTT. Then export the hidden features to another image and put that on a "GM layer" overlay. Once you have the process down, it's pretty quick to draw your own maps -- but still better to use one of the many hundred thousands of maps out there; the work is already done for you!