The Ambleside Online curriculum supplies you with a booklist and a schedule for all the readings, so most of your planning for each school year is already done. However, you must still do some planning of your own to prepare for the coming year.
Start by reading through the AO FAQ. If you've already read it, skim through it looking for areas you may not recall well or that you may need to do this year but have not done in the past. The FAQ will help you with implementation details.
Next, check the booklist for the year you'll be using and make sure you have all the books. If there's a free copy of the book available online, the booklist will usually link to that.
Then, you need to tweak the weekly schedule to fit your scheduling style and to include all the extra areas you want to cover each week. I do not try to plan by the day. Instead, I try to have a plan for how much we will cover each day, but then allow my dc to choose or help to choose exactly what we do each day within those parameters. I've tweaked the schedule to facilitate this in two different ways.
I've copied the weekly schedule text right off of the website and pasted it into a word processor, added areas I wanted to include that weren't listed (math, artist study, etc.), then added little blanks (_______) next to each weekly item and multiple blanks (____ _____ _____ ____ ) next to items that need to be done more than once a week. That made a nice checklist that allowed me to easily see what was already done for the week and what remained.
I've also used a chart format schedule (watch the Ambleside website for new, revised chart format schedules). I add rows for the extra subjects I want to include, delete rows for the options I'm not using, and add checkboxes to items that need to be done more than once a week. (I use a large letter 'O' for this. I put several of them into each weekly cell of the spreadsheet.)
(I've blogged about my schedule formats before.)
After I have my schedule set up, I look for any extra resources I will need.
I print the art selections. (I have mine printed in color at a local print shop. I upload the files through their website, place my order and pay online, and simply walk into the shop to pick up my prints when they are ready. I prefer to use the prints from the AOArtPrints Yahoo group.)
I make a CD of the composer selections. (I use Classic Cat to find free and legal downloads. If I can't find a free download, I look for an inexpensive one to purchase from Amazon or iTunes or eClassical. Sometimes I even purchase a CD if I can find one with all or most of the selections, in their entirety, for a good price.)
I look for YouTube videos of the hymns and the folk songs we'll be singing, and I print the lyrics.
I think about what geographical areas we'll be studying for geography and history, and I print maps for those. Sometimes I use generic maps and sometimes I do a search for more specific maps for that historical situation.
The Ambleside forum often has links to maps and other resources for various books. I look through those links for resources we might want to use, and I look in the Files section too. I also save emails from that email list when they have references to resources or advice for future years. I put them in email folders designated by year, and before each new year I read through those for ideas. (If I didn't save the emails, I could search the message archives for the same type of information.)
I usually put everything I print out into a notebook for the student, divided by subject. That way most of our material for the year, apart from books and manipulatives, will be in one place for easy reference.
And then I'm done!
(Updated 7-Apr-2013 to change references to out of date resources.)