Creating a Schedule!

I need help. I desperately need to create some type of schedule for our daily activities. I never know day to day what we will be doing or when we will do it. This makes planning for anything almost impossible. I just plan things and hope for the best. I don't know how many doctor visits I have had to reschedule due to kids taking naps late in the day. I don't care if the schedule is perfect I just need some type of a guideline. However, I don't know how to do this I have never been on time for anything let alone follow a set schedule. We have always just let our kids tell us when they are hungry or tired and worked around that. For infants this strategy is fine, but as kids age and become more vocal and active this no longer works. They become over tired and can't sleep or are really cranky. I would like to avoid these tantrums, and show the world the well behaved kids I see when they are not hungry or sleepy. My dad says my daughter is always cranky, but he never sees her when she isn't tired or hungry. I don't know what to do or how to do it. I think for my kids this is a necessity, but I am at a complete loss. I think life would run smoother if the kids and I new what to expect next. How the heck do you do that? Any ideas!