Choose an image. Whether it's large or small, complicated or simple, it helps to have a reference in front of you.
Break down the image into its basic parts, remembering that there's no wrong way to do this. If you're trying to make a truck, for example, look at all the different circular shapes on the keyboard you could use for the wheels: O, o, (), @, 0, etc. Look at the image as though it were already made up of letters. The front of the truck could be represented by "L" or "[." Don't be afraid to get creative!
Check your alignment. You're likely going to have to use multiple lines of text, putting things on top of each other to create larger pictures.
Remember that font type and style are going to impact the overall look of your picture. Feel free to experiment with various sizes and font types for your image.
Look at examples of other keyboard art. By doing an Internet search for "ASCII art," you'll find tons of pictures and websites by people who've made this their hobby. Find common ground in the pictures; some use symbols such as # repetitively to represent blank space, while others use simple lines to make simple pictures.
Experiment with color. Most ASCII art is black and white, but it doesn't have to be. If you feel that the content of your picture may not be coming through the way you'd like, change the color of the individual letters to represent parts of the image.