The first thing you do is brainstorm. Write down the basic point and objective of your sceniro. Then, write down the main characters. Draw out the map on a piece of paper. Then, go to your map and lay out the basic resources (trees, gold, etc). This is important because even in RPGs, there was still gold and stone mines, even though most campaigns forget these. Next, lay out the cliffs and elevation. Don't use the same elevation throughout the whole map, that will look too plain and unrealistic. Make some cliffs, lakes, and trees. Next, add the paths, flowers, forage bushes, rocks, and other terrain things, like flags and stuff. Now, add the buildings. Mix in your eye-candy as you go along. A good eye-candy can be the city if you take enough time to design it. Next, add the units. I generally do gaia (deer, hawks, etc) first, then player 1s, then allies, then enemies. And last but not least, triggers.
1.) Brainstorm
2.) Terrain
3.) Buildings/Eyecandy
4.) Units
5.) Triggers