Depending on the track, I'll either start with a set of chords that match the mood I have in mind (often in arpeggio form), or I might start by trying to find two or three sounds that match the mood I'm going for. Occasionally I'll start with drums and bass particularly if rhythm is the focus or if I have a less common time signature in mind.
Usually I'll start on a piano VST or Jup-8 (because I love that synth!)
While away from my computer, I write any ideas (chords, story, instrumentation) that come to mind into my notes app (for melodies I'll hum them into my recorder app). For this jam in particular, I came up with way more ideas than I could use lol.
Of course I always have the image theme open on my second monitor.