Usually human cells accumulate random genetic mutations over time that get passed down to other cells that they divide into. Each cancer is different but there are some mutations that are "the usual suspects" to look for. For instance mutations in the p53 promoter region will disrupt the cells ability to self destruct when it detects mutations. Obviously that contributes to the likelihood that it will become cancerous. This way of doing things is fairly similar to the way in which cancerous mutations would arise naturally in humans, but it is much less random.