Decorate a notebook or a store-bought journal with drawings or even a painted cover. You can attach beads, feathers and other decorative elements as well. Make it personal. An art journal should be visually appealing.
Think about a concept that interests you. It could be something as deep as world peace or as simple as lying on the beach every day. Sketch a symbol or an image that represents this concept. It can be a simple pen illustration or a complete color pencil drawing. It can be literal or abstract. Take the sketch as far as you want to go with it.
Write a title for your drawing below the drawing itself and then write a paragraph or two that describes what the drawing means to you. Like any journal writing experience, you're expressing observations and your own thoughts and feelings. Label the entry by date.
Link your entries together with a theme. Work from your initial concept on each entry so the journal comes together cohesively. Not every page will need image and text, but try to use a combination of the two whenever possible. Try alternating between literal and abstract representations of your concept. The purpose of an art journal is to find words that represent images and images to fit your words, so start each entry with whichever of the two elements strikes you first and build the other element around it.