How can I breath effectively when walking?

You have to find a breathing technique that is comfortable for you and you need to focus on that technique from your first step to your last.  The easiest technique will be to take deep breaths in through your noise, exhaling through your mouth.  This allows air to circulate in through your lung and be exhaled through your mouth.  Breathing in through your nose is beneficial to your body as your nose will restrict from too much air passing through your body.  This will also assist in breathing patterns to help keep your breathing controlled.  Also, be sure to walk upright with a good strong posture.  This will allow you to breathe easier and more fluently.