I am doing the Marceline to Magic Kingdom tour in April and I booked it in November, so about 5 months ahead of time. The day I wanted was already full, which surprised me, but I was able to easily adjust my plans to do a different day. I would book it as soon as you can, just to be safe. Especially if you need a specific day and your plans aren't very flexible. To book it all you have to do is call Disney, the number is one the tour's page on the Disney site. Just let them know what tour you want, what day, who's going, and some other basic information. It is pretty easy.