Decide what image you'd like to make. Whether it's a flower, a duck or something else entirely is up to you, but having an image to reference will help.
Look at your image as though it were already made up of keyboard letters. If your image has spots, they can be represented with o's or @'s, or even ()'s. Stripes can be /'s, or x's. This is art; there's no wrong way to do it.
Make use of paragraphs and rows to put things on top of each other. An _ with an | above it will have the same effect as using an L, but bigger. Experiment with multiple lines of text to create an image.
Find examples of similar art. There are an infinite number of ways to make the same picture; looking at what someone else did may give you your own ideas and inspiration.
Use color. If you don't feel that your flower looks like a flower, turning the stem green and the petals pink will help it come across better.