I think there are 2 main stages - general planning, and then scheduling. The general planning takes the longest. (It took me 3 years to find a Canadian History spine for years 3-5 that I felt met the CM standards, and was available for free.)
First I look through the AO year that I am working on next. I try to do this over a year in advance, as this can take a while. What books are very American, that I might want to take out? What time period are we in?
For each book I decide to replace from AO, I do some looking around to find an appropriate replacement Canadian book. For example, in Year 3, I knew I wanted to replace Squanto. It was American History (not to mention out of print.) I determined it was placed in about the 1690's. So I hoped to find a good Canadian History book in around the general time frame. Obviously I can't always meet all the criteria, but I hope to find something around the same length, with a similar number of chapters, similar difficulty. And it should be a living book. And my preference is for it to be something that is available free, or is available at our library, or is something we already own.
I try to find history books - but I also try to find Canadian literature as well. (I hope to do more Canadian artists, music, etc as well... we have done some, but not too formally at this point.) I try to have at least one Canadian literature book each year....
By this point, I should have a good idea of which books will be replaced, and which books are being added. I copy the AO weekly schedule for the year into Microsoft Word. I do a bit of formatting changes - mainly taking each list for each week, and change it to a numbered list. (I do this to help me know how many readings there are.) For AO books that list a reading with several chapters together, I separate those readings to a reading per chapter.
I then look at the first book I am replacing. If the number of chapters/readings are the same - then I just replace the one book with the other on where it is scheduled. If the number of readings are different (or it is a rarer added book) - then I look at the number of readings for the book I'm adding. Is it something that will go nicely into the year (or term, if for only one term?) For instance, I determined that the History Spine Readings for Year 3 (Child History of the US, and the way I'd broken down Story of Canada) came up to 18 readings over the entire year. This would fit nicely for every other week. I celebrated that one as the Marco Polo book I chose also had 18 chapters - so we alternate a Canada/US reading with a Marco Polo reading. Some patterns can be a little weird. 20 chapters over a term, for instance, go along the lines of 2 chapters, 2 chapters, 1 chapter....
So now, I have removed all books we won't be doing, and have "pencilled in" the new books. I now look at the number of readings each week - which is easy because Word numbered them for me! The number of readings generally isn't the same every week - but is generally within a couple. Year 3, for instance, seemed to vary from 8 to 12 readings. I look at the heavier weeks (the 12) and see if something could easily be moved to one of the lighter weeks (the 8).
A couple of other things I do - I use a different coloured font (blue) for the new books I put into the schedule, so I know at a glance if something was from AO or from me. I also go through and highlight (yellow) the first time a book is used that we need from the library. The weekly lists that I have as a result are put in my children's planners. When they do a reading, they cross it off the list.