Preparation usually consists of two stages. The first stage -- call it "pre-preparation" -- is performed ahead of time, producing a non-perishable but not-yet-edible form of a complete food (usually a powder). The second stage -- call it "post-preparation" -- is performed shortly before the time of consumption, and it turns the pre-prepared complete food into an edible complete food.
Generally the pre-preparation is a more complex and time-consuming process. It involves measuring out correct amounts of all the ingredients. mixing them together, and putting them in a storage container. That may be all it involves; or it may also involve soaking, cooking, freezing, or otherwise manipulating the ingredients.
The post-preparation is, ideally, a fast and simple process. It generally involves adding and mixing in water. That may be all it involves, or again it may have other components.
The most typical preparation process, whose goal is a liquid complete food shake, is as follows.
All ingredients are measured out into a bowl. Maybe solid and liquid ingredients are measured into separate bowls. For ingredients measured by mass, a sufficiently precise scale is required. For ingredients measured by volume, appropriate measuring containers (tablespoons, teaspoons, measuring cups, etc.) are required.
The solids are mixed thoroughly, with a whisk, an electric mixer, or a spoon. The liquids (if measured into a separate bowl) are added, and mixed in with a whisk, mixer, or spoon. The pre-prepared complete food is then transferred to a storage container.
Some of the pre-prepared complete food powder is mixed with water, and the two are mixed using a blender or a blender bottle. Most typically, this is done at the beginning of the day with a blender, and it is done to the entire day's complete food. This may take multiple blender loads to do. Most of the complete food is stored in the fridge (if this is not done it might go bad by the end of the day), and some may be carried along with the user in a bottle. The user should give the bottle a shake before drinking.
A common question is, what is the appropriate ratio of complete food to liquid? The complete foods team recommends that a day's worth of complete food be mixed with 2-3L of water. This produces a milkshake-like consistency. Smaller amounts of water can also be used to produce thicker consistencies; any amount of water is acceptable, all the way to the point of producing solid complete food. The total daily water intake for a normal, healthy individual is 2.5L-3L, This includes water from all sources. If you perform lots of strenuous exercise, work outdoors in an extremely hot environment, or have a medical condition, you might require more (or less) water.
To produce a solid complete food, the simplest way is to use less liquid. If your recipe has a lot of moisture in it to begin with, so that your mix ends up being a paste rather than a powder, then you may not need to add any liquid.
If your recipe is a powder, then it needs more liquid to become an edible solid. Proceed as follows. Add water to the mix gradually while mixing. When the powder begins to form into clumps, it is close to becoming solid. At some point it will become solid, and you are done.
Other people have come up with creative preparation processes. Particularly unique and instructive is J. Jeffrey Bragg's preparation process, described in this thread:
In a quart-size glass jar, combine the oats, buckwheat, spelt, cocoa powder and peanut butter powder and mix together thoroughly. Add the yoghourt whey and enough slightly warm water to wet the dry ingredients well; a quart jar will be perhaps two-thirds full. Set aside and let it all soak for at least 24 hours at room temperature.
When the premix has had its thorough soaking period, empty the jar into a saucepan. Mix in the dry egg solids add just enough water to ensure liquidity (bearing in mind you need to add more water to the dry milk powder and that the final result quantity shouldn't go over 8 cups) and cook over very low heat, stirring quite frequently to avoid any sticking or burning; a very low gas flame is best. Bring it slowly just to a boil and simmer, stirring almost continuously, until the cereals and egg form a thick gruel. It wants to cook maybe ten or fifteen minutes.
Remove from the heat and let it cool a bit. While it's cooling, put the dry milk powder into the VitaMix with a couple cups of water and blend; let it stand awhile for the reconstituted milk to blend. Add the coconut oil and the olive oil to the cooked gruel and stir them in well. Then add the other dry ingredients, the molasses and the syrup. Cut up the banana without peeling it and toss it into the VitaMix with the milk, and liquify the banana. Finally, add the dark brown gruel mixture to the milk/banana mix and process (with the lid on!) on high -- be careful, blending the thick stuff in is a challenge even for the powerful VitaMix and if you aren't careful it may engage its automatic protective cutout, in which case you'll have a long wait on hands before you can finish the job.
The resulting complete food will be THICK, thicker than a milkshake, more like the Dairy Queen frozen-yoghourt Blizzard used to be before they quit selling it. You can consume it on the spot, or put it in the freezer! I do the latter and partially thaw it in the microwave as needed -- but I don't live on complete foods. For me at the moment it's still an adjunct to a regular healthy-foods diet, something I use when I'm too tired to prepare a meal, or I take in the truck when I've got a long afternoon of driving ahead of me. You'll need a soup spoon to eat this stuff.
Also instructive is the example of Nicole Cannavaro's porridge:
A lightbulb went off today in my head when I was about to pour my oat bran into the blender. People all across the world eat oat bran as a breakfast. How would it taste if I blended the liquid vitamin and oil separately, and mixed the powders into the hot cereal? I thought about this more, knowing that texture-wise, 300g of maltodextrin to 100g of oat was probably not going to have a very good consistency, and taste wise, it may be even worse. Thus, I decided to cook up 200g of oat bran, and added in 200g of maltodextrin along with the other powders and capsules, mixing my oil with my multi-vitamin and drinking it on the side.
The results were something like a cheesy romance film meets the Food Channel – love at first taste. Within the first few bites I knew I had hit the jackpot, and by the time I finished the bowl I was dreaming of all the lovely years that my new-found ‘super porridge’ and I were going to spend together.