If you get the chance to do a nice thing for a person, then do it.
That sounds easy. Just do a nice thing when the opportunity presents itself, and move on with life afterward.
It could be a friend. It could be a foe. It could be a total stranger who would have no idea who performed the deed.
It should be anyone who happens across your way at whatever random moment of the day.
Seriously, just be nice. The world will be so much better for it.