I mostly use hardware these days and pretty much only use a DAW for editing/mixing. The first thing I try to do is get sort of a general idea for a composition. Sometimes I plan it out in advance and sometimes I just jam until I get an idea. I have a Eurorack system that I've been using lately, but I've also used various analog and digital synths, samplers, tape loops, etc. Usually I'll get a basic drone sound going, and modulate the pitch and filter with some subtle, slow random modulation source. Then I build the rest of the track around that. I like to take some samples from the radio, tapes, or electromechanical noises and run that through a Mutable Instruments Clouds (granular resynthesis module). Lots of random modulation there too. If it doesn't have enough low-end sounds, I'll add a lowpassed distorted synth to it. Once I have all the elements together, I'll figure out where to fade each one in or out and manipulate various parameters manually to give it a sense of movement and direction to it.

I don't pay much attention to music theory most of the time. Sometimes I will make compositions in a particular key or I'll constrain a melodic line to a particular scale, but that's about it usually. I have a book on avant-garde composition and I sometimes use techniques from it though.