Light the candle on your krathong, set it in the water, make a wish, and release into the current of the river for it to drift away.
It's that simple...
Unless there is a troll under the bridge stealing your wishes.
The Loy Krathong festival celebrated in Thailand each Oct/Nov is especially prominent in Chiang Mai. People make, or buy krathongs, often made with banana leaves on a tree core (or more regrettably foam). They are then adorned with flowers and candles to be released into the river. Many will also include a small amount of money as an offering as well.
In a perfect world, people would release their krathong into the river and watch as they float down stream, the candle on top flickering in the distance until it is lost in a sea of other orange dots.
Having made and released my own krathong in 2011, I came only to witness rather than participate this year. While crossing the bridge over the river late in the evening, occasionally dodging wayward fireworks launched from the shore, I spotted this kid in the water under the bridge. Less than 10 meters downstream from where many people were launching their krathongs from a crowded low spot near the river, he was camped out in water up to his ribs in the chocolaty waters.
He was stealing wishes.
With an eye out for any baht (Thai currency) that might be making the journey on the top of the krathong, he positioned himself right where they would be passing within his reach. To add insult to injury, after pillaging what he could, he would often play target practice, picking up what was left and tossing it out in the river towards others floating out of his reach to take them out.
Check out the short video I shot below to see him in action, and next time you take part, beware of the trolls under the bridge.