Mindfulness meditation is a process of letting go; letting go of the way you think things should be and accepting what is. Often this can be hard when we start out, especially if we've been told that we need to let go, live in the present, and stop fretting about things. The harder we try, the more confused we get. The way to do it is to allow it to emerge - to focus all of your intent on simply being, not straying from being aware of the content of your own experience.
Living mindfully in a city requires a different approach to living mindfully within nature. A city like London is actively trying to grab your attention; every headline you see on an abandoned newspaper demands you make a judgement, advertisements shout for your attention and the constant stream of people and traffic is an assault on your senses.