"and to put on the new self, created to be like God in true righteousness and holiness." NIV translation
In Genesis 1:27, we are told that man was created in the image of God. That image was distorted due to sin but, through Jesus Christ, that sin is removed. Not only are we to get rid of the old self but we must replace it with the new. In Matthew 12:43-45, Jesus told us what would happen if we did not put on this new self.
"Therefore each of you must put off falsehood and speak truthfully to his neighbor, for we are all members of one body." NIV translation
When you put on the new self, there will be some big changes in the way you interact with other people especially other believers. This first practical part of learning to be the light is speaking the truth. As children of God, our words must reflect the Word of God and that means every hour of every day.
James 3 gives us more instruction concerning the need for taming our tongues.
"'In your anger do not sin'. Do not let the sun go down while you are still angry, and do not give the devil a foothold." NIV translation
Paul quotes Psalm 4:4 which was written by King David as he faced persecution. This passage is not telling us that we can't get angry but it is saying we must remember to forgive. When we do not forgive, then the hurt or anger hardens the heart and opens a door for the devil to work against us. The Jewish day started at sundown and so he is saying don't start the new day with leftover hurt or anger from the day before.
"He who has been stealing must steal no longer, but must work, doing something useful with his own hands, that he may have something to share with those in need." NIV translation
Another part of being the light is actually doing some useful work. There are many people doing many things in the name of God but we must ask ourselves why we are doing them. We see here that the reason that we work is so that we can share with those who are in need. That is a great way to be the light in this dark world. If we are working to build up things here on earth for ourselves or even for our families, that is not the same thing. We must each ask ourselves (and truthfully answer) this question of why we are working.