So let's walk through yesterday, and to make this a little easier, imagine your favorite movie star in the starring role of a movie of yesterday (that way describing the good parts probably won't be quite so difficult). With whom did you interact? How did those interactions go? Did you exemplify the love and grace and truth of Christ in any of the interactions? How? Did you go to work? Why? Did you remember to pray? When? Did you remember to just talk with God when needed? Did you take time to listen to people who needed an ear? Offer a shoulder to cry on, followed by a tissue? Did you offer a smile to someone who's day seemed to need it?
Now the other side - but this part's a bit more difficult too, because you're being asked to ensure that it's not mistakes that you're listing, but intentional actions or words that help break a relationship between you and God, between you and another. List where you purposefully said something unkind or went against what you know God wants you to do.
Take a look at your list now. And know that the Grace of God will always be much stronger than the sin of man.