I'm sort of in line with GP's thoughts. Although, as others have mentioned you might need to ask a more specific question to get a better answer for you.
I have gone both ways. What I will call "outside in" and "inside out". What do I mean? Well, the first means start with the landmasses and move inwards the other means start at a location and move out.
Outside In
The planet doesn't care about the denizens, it makes it's form and people just naturally gravitate to the best places. Ie, fresh water, lots of game, various industries. Imagine you are the first colonists, where would you want to build? and then just keep adding colonists and imagine the cities they would have built depending on the opportunities provided by the nature around them. Keep asking yourself, "Why does this city exist here in this spot"? As in business, 'location, location, location' means everything. Mining towns existed because there was a valuable mine nearby. Most also failed after the mine was depleted, unless they had another value added reason.
When doing this type of map I try to create the best of what I consider "cool" looking continents and go from there.
Inside Out
Here I've got a specific location in mind. This is how my Homebrew game map has grown. I started with the location I wanted my players to begin and expanded from there. I imagined a Keep on the side of a mountain with a river flowing thru the valley below on it's way to a distant sea. I had to add mountains to help explain why the river flowed the way it did and forests needed to be nearby for timber. I made sure the mountains divided them from the more civilized cities because I wanted them to be out in the middle of nowhere so they have a lot to deal with on their own. That provides all sorts of gaming opportunities. Everything then moved into cultures of my the world, the various races. Dwarfs beneath the mountains, a city of thinkers and so on and on and on until I have what I have now and yet, It's still not finished.
IF on the other hand you mean how to you come up with ideas for what a city might look like based on it's technology, well that's a different matter.
Start with references. Grab a snapshot of a huge organ and use the pipes for an idea connected to some sort of engine. Honestly, this is a rabbit hole that never ends, just grab any image that inspires you and then draw a kobashery (made that up) of bits and pieces as a new whole to build crazy ideas. Get like 10 images and turn them into 1 somehow and see what you get.
Anyway, that's all i got for now. Hope it helps.