Quoted from my answer on Quora:
I open Notepad (on a Windows PC) and start working on an idea. It’s like a puzzle to fit the characters together in a way that creates an image.
Some inspiration comes from searching for images or those I find. I look for clean lines so it will be easier to adjust when I need to figure out the spaces, characters, slopes for lines, and so on.
The font you work with makes a difference. Notepad uses FixedSys (or one of the newer offshoots). I seldom like how ASCII art looks when it is shown in another font. Courier and others use a thinner line which makes the ASCII art seem washed out. Plus, even though they are monospaced, the lines in columns don’t match up exactly the same.
I know some people would make a pattern or sketch and tape it to their monitor. So the ASCII art would be like filling in the lines (connecting the dots). I’ve never done that. Most often I have a general idea and add some characters, sit back a bit and see how things are shaping up as I go along.
Don’t delete a bunch of stuff and start over too often. Sometimes just adding a bit of detail will pull the whole image together. Circles and curves are one of the hardest things to work with, especially if you make a smaller ASCII art image. Close lines, like trying to make fingers for a hand, are pretty much impossible unless you make a bigger image or settle for a hand with just three fingers. I avoid hands and just have people holding things. This works well cause it adds more details to the image and explains what it was meant to be.
I don’t use any alt characters. ASCII isn’t about anything but the standard keyboard characters. You can do a lot with lines and dashes but I find a way to use numbers and letters too. It seems a bit bare without them.