"Blind of the fact this topic is concerning CPE theory...."
When I begin a work, trivial or serious, I first start with a series of pitches (with or without rhythm) -usually between 2 to 5 pitches for a motif. I expand the motif by adding another 2 or 3 pitches at a time (sub-motifs). Depending on the idiom (serial or non), I may use a matrix to help organize my material OR just begin writing. The idea here is to provide focus on brevity. People love things they can remember -and they especially love things they can easily pick up on. Keeping your material small allows for you to optimize and maximize this. I hope that makes sense.
After I build the motivic units together to create a musically logical (or illogical, lol) sentence. I look at the potential areas that I can take this. Do I want to develop the material further? Does the idea scream for a short work that leaves the listener curious? How do I want the idea to evolve and flow? Once I have an idea where I want to take the material, I begin to select the proper form for it. Forms, as I mentioned in another post, are another aspect of theory that are extremely important. The form allows you to state your ideas, develop them, and allows the listener the opportunity to follow your ideas through the process itself.
Notice I've not talked about harmony? Despite my education and attendance in music harmony courses... I would argue that harmony is an item of music that is the byproduct of these other processes. Obviously, in developing the motivic units and the overall sentence of my ideas... the harmonic language (and subsequent rules associated with the language) should follow suit. But these rules and language -in my personal opinion- aren't meant to be followed to the fullest. After all, what if the idea behind my motif was to cause my listeners to question their own socio-cultural understanding of music??? At any rate, harmony to me isn't as important an element as the construction of your ideas or the form in which you plan to develop those ideas with.
So, to summarize, the list of things to be aware of when embarking on composition are the following:
1. Motif and submotivic units
2. Form
3. Your musical language.
I hope this helps!