First you need to consider the time it takes to make each component, the shelf-life of each component, and what components need refrigeration.
Gumpaste flowers take multiple steps to create, and hours (sometimes days) of drying time between each step. Thus, you will want to start on these days in advance. Make more than you think you will need. Some will break; some you will like more than others; some may be needed to hide problem areas. Some fondant details can be made a day or two in advance as long as they are stored properly. I could have cut the eyes and the swirls that make the mane, and stored them on a plastic lined sheet tray, wrapped super well with plastic. If you are using perishable fillings that need refrigeration, you should prep as many decorations in advance as possible. Some decorations should not be refrigerated (gumpaste and most fondants). Although some fondants are created to withstand high humidity and moisture changes (Carma's Massa Ticino Tropic!!!), moisture and temperature changes from your refrigerator to room temperature can cause sugar to sweat, which will can make your colors sweat and your fondant sticky.
For fondant covered cakes, I usually bake all cakes, and make all the fillings, 2 days before the due date. This gives everything a chance to set and cool before any assembly or carving is done. Remember, you CANNOT fill or frost a warm cake.
To help illustrate the process a little better, I decided to break down the Unicorn Cake for you...also because a lot of you have been asking.
Day 1 (Wednesday)
- Color all gumpaste and fondant.
- Create flower "centers" and allow to dry 4-5 hours
- Shape and assemble flowers and dry 24 hours
- Shape "horn" and set to dry 24 hours
- Bake Chocolate Cake
- Make Mascarpone
- Make Raspberry Preserves
- Make Buttercream
- Make Rice Crispy Treats
- Finish Mascarpone
- Assemble and sculpt unicorn
- Assemble base
- Cover cakes with buttercream/crumb-coat and chill
- Paint and finish all flowers and "horn"
- Cover cakes with fondant and finishing details
- Final assembly