In order to practice a walking meditation you must first find a quiet place with little activity. The length should short and your pace should be on the slower side. Before you begin take a few moments to focus on your body and anchor yourself in the present moment.
Now simple follow the sounds of your footsteps or you can focus Lon your breath or nature in general. As long as if you find a focus point and are fully focused and your thoughts are not wondering then you are doing a walking meditation.
Buddhist monks used to use the walking meditation after sitting for hours while meditating. I myself use the walking meditation on a daily basis as part of a way to incorporate movement meditation Into my practice.
Here are some more advanced walking meditation techniques you can try.