Self-injury mostly happens in private. Usually, it's done in a controlled manner or the same way each time, which often leaves a pattern on the skin. Examples of self-harm include:

Cutting, scratching or stabbing with a sharp object, one of the most common methods.
Burning with lit matches, cigarettes or heated, sharp objects such as knives.
Carving words or symbols on the skin.
Self-hitting, punching, biting or head banging.
Piercing the skin with sharp objects.
Inserting objects under the skin.