Logically the next way to begin would be to use no plan. To compose freely and respond in the moment to sounds you hear. This is much easier said than done. For some this method works well while improvising. It could be done in a group or individually. It could be on any instrument the composer feels comfortable with. For others it could be done using music notation software that plays back what you have written. The important things to remember are:
If you are improvising, you are improvising with a purpose; to compose music that others can replicate. I very often use this method at the start, to ‘get pitches’ but then abandon this method so I can focus on developing ideas on paper. I find there is a big difference between what comes to me in the instant of improvisation compared with what comes out on paper.
When we use this method it is important to remember all the aforementioned characteristics of TONL. To help keep this in mind I keep a somewhat largely written version of the row in front of me. I have also had great success when working with other musicians as well and employing this same method. When I do this I most often record the session and then let the recorded material rest a while (maybe an hour, maybe a day, maybe a week, a month, a year) and then I go back and listen to it, keeping an ear out for interesting material that came about. Then I transcribe any pieces of interest and keep a little book of these ideas. Below are a few examples of self improvisation on the piano, guitar, and keyboard.
Finally, I should mention one last method of improvisation that has borne great fruit. This method requires that you have a way to multi-track record. In other words, you need to be able to record yourself improvising with yourself. To start, I might record a bass track. I would keep the row infront of me while recording and use it as the inspirational pitch material for the music. Then I will layer another track on top of this one, perhaps a keyboard. You can place other limits on your playing as well. Sometimes this helps to prevent feelign like there are too many options or too many directions that the music can take. After recording a few tracks, I will let the music rest for a while, and come back with a fresh ear to see if anything good came of the session. The key is to play non-judgementally, and to make mistakes often.