"I’m so disorganized that I have trouble using the day planner I just bought. Any suggestions?"
Your goal should be to carry your planner with you at all times (or at least know where it is), and to check it three times a day — when you wake up, at midday, and at bedtime. To form this habit, try playing a little game with yourself for the next week. Check your planner every time you get a cup of coffee or drink of water, answer your phone, and go to the bathroom — even if you have nothing written in it for the day.
Checking your planner this frequently is overkill, of course, but you’ll be surprised at how many reminders or to-do items you think of to write in it. Soon, checking and using your planner will be second nature.