Writing your own music is not as difficult as you think. As with most things, it requires some discipline and forethought. It is definitely easier to play your own songs than it is to play songs you've learned from books or friends because while creating the song you improvise on your own skill level.
You may also think that to write your own music you need to know all kinds of music theory. But music theory, while essential for understanding musical construction, isn't really required for creating music. Music theory puts a logical structure on music which allows us to analyze and categorize it, but by its very definition, it can actually hinder the creative process. Its a left brain function (verbal/analytical side of the mind) and the creative process is a right brain function (spatial/holistic/artistic side of the mind). If you go into the creative process without an analytical mind set, you are free to find a musical expression "outside the box"; ideas you may not have discovered otherwise.
Now that isn't to say that music theory isn't important...it is. In fact, the more theory you know, the better, but the more you can free your mind of it while creating, the better....just don't use to do it :) I will point out below were you could use it to your advantage.
So let's give it a try. As we go through the steps, I'll provide an example to help illustrate each concept.
01. First we need to decide what tuning to create in.
Standard tuning can be a good starting point, but since this is an Open Tuning tutorial, I propose you try something in an Open Tuning, like DADF#AD. For one thing, I think you'll find it easier to explore new musical ideas in an Open Tuning.
One reason for this is that open tunings are optimized for a particular key and the music can resolve itself to open strings in that key. Writing music in standard tuning usually requires that you fret every note and open strings are generally used sparingly. This usually means you need to think more about the key you are in and where you want to take the music; that music theory thing again. In Open Tunings, the opposite is frequently the case, and in fact, the open strings can offer some surprisingly refreshing sounds.
02. Most (if not all) songs are built from sub-parts. Step two is to build these parts.
This is were your creativity comes in. Explore the new tuning. Some of your old fingerings may work, but will sound very different. See if you can "discover" five or six chords that you can use as basic material; but try not to restrict yourself too much with this. Use these chords to improvise from. Let your ears be your guide. If it sounds good to you, go with it.
The goal is to find two themes of about two or three measures in length that are different but have something in common in terms of atmosphere. We'll call them A and B.
03. So far so good. You can now use A and B as building blocks for your song.
To create something that keeps the listener on his toes, you will need to expand these themes. Again, start improvising and create two or three variations on these themes. The variations can be very small, a note added here, an inverted form of a chord, a rhythmic variation, a transposed form, whatever sounds o.k. Try slides, pull-offs and hammer-ons and see whether they add something. Approach one of the central notes in your theme in a different way. Do the same with Theme B until you end up with two or three variations of A (A1,A2,A3) and two or three variations of B (B1, B2, B3). Now pick the theme that has the potential to serve as an intro.
04. Setting the structure.
The structure of a song is very important; it determines its emotional impact. One form that works very well is A1-A2-B1-A1-B2-A3. Again the form you select will depend on the material you have. The last part should have enough power to make a convincing statement. In some cases the last will be derived from A in others from B. You may need to experiment with different forms before finalizing your structure. Some of the themes will fit together effortlessly, others may need some bridge notes to make them fit. Another way to improve the result is to watch for tension and release moments in your themes. If these moments are lacking, you may have to adjust some of your themes to create more contrast and more of a question and answer feeling.
05. Capture or memorize what you end up with.
If your song has the right length, you are ready to finalize it. If not, expand the structure to stretch it. One way to do this is to repeat the first section once: (A1-A2-B1-A1) repeat B2-A3.
If you don't already have one, you should seriously consider getting yourself an inexpensive Tab software package like Guitar Pro or TablEdit and capture what you've settled on. For one thing, you'll find that it's easier if you don't have to keep the whole thing in your head. It's not at all uncommon to come up with a theme during one session, and find another theme to go with it a week or a month later. I frequently combine themes that were "discovered" months apart. Before computers and TAB software, I couldn't begin to tell you how much original work I forgot because I didn't take the time to write it down. Having it on paper and tweaking a section here and there is considerably easier if you can just call it up on the screen and make the adjustments to what you had. Once its on paper or on disk (with backups!), you can store it away and not worry about forgetting it.
Open Tunings is whole new frontier that will give you an almost unlimited palette to draw from in your creative exploration of fingerstyle guitar. I hope you've found this process useful and I would be interested to hear what you come up with.