I blog elsewhere and do this a bit on the fly, but it comes together as you work it.
Keep a list of possible posts - add to it constantly.
Schedule out the best for a given week and get those going
Pull the second out for week two
Keep adding ideas and producing the content and eventually you end up with a month, quarter or years scheduled - depending upon what you do.
For example, I am working on Halloween right now for some of my places. Christmas and associated holiday posts are coming along. I do weight loss, so I'm also working on my January posts now as well.
A second example - I post 2 "inspirational" posts a week in one place. I have enough of those posts ready for months to come. I make them up as I see them and sometimes move them around a bit - but they are there when I need them.
I hope this helps :)