How do you plan an entire year at once?
I don't. It's more of a loose, this is what needs to be done this year and this is what I'd like to do this year sort of thing.
I usually do my yearly plans in July and plan out the year for August through the next July. This is mostly because my family is on a school-year sort of schedule and because our years tend to flow that way. In January I will often do some goals lists.
So really, How?
I usually take a piece of paper and write down the months of the year. And then I make note of important dates, appointments, holidays, etc.
Then I make a list of things I want to accomplish and divide it into little bits that can be done each month. This was a big part of how I planned my home-school years. When you do things in small steps you can accomplish so much more than you thought you would.