Willy's fall from grace in his eldest son's eyes can be traced back to when Biff goes to Boston to visit his father and he discovers him with the Woman. This is what changes the course of Biff's life, as he returns having no interest in passing Math and therefore misses out on his college scholarship. Note how Bernard makes reference to this in Act II:
And he came back after that month and took his sneakers--remember those sneakers with "University of Virginia" printed on them? He was so proud of those, wore them every day. And he took them down in the cellar, and bruned them up in the furnace. We had a fist fight. It lasted at least half an hour. Just the two of us, punching each other down the cellar, and crying right through it. I've often thought of how strange it was that I knew he'd given up his life.
Therefore, based on the information contained in this quote, it is obvious that the defining moment in Biff's life is the discovery of his father with the Woman. You therefore have to choose when you want Biff to write his diary: will it be straight after this discovery, or will it be when he reaches adulthood? Either way, a diary entry will have to talk about Biff's feelings for Willy before this discovery, then the discovery itself, and now Biff's feelings about Willy. Using this three-stage structure should help you to develop your ideas. You might want to look and see how Biff pre-Boston talks and relates to his father to get some idea of the devotion he had for Willy. Good luck!