The main thing is to record any idea that come into your head, e.g. first thing in the morning when you wake up, right away. Sing it into the voice recorder on your phone if it's not convenient to pick up whatever instrument you mainly use. Regularly listen to all these bits of songs and see which, if any, fire your imagination.
If I'm struggling with lyrics, sitting on the toilet with a sheet of paper on a clip-board works for me almost every time. Maybe because there are no distractions?
I also listen to songs that are "complete" and mastered fairly regularly and take notes. Sometimes there is a slight mistake or "something missing" that can suddenly lift a song from "ok" to "brilliant" once you add it to the mix then remaster it. I heard a quote that said "if you think a song is ok, then it isn't". Ok isn't good enough. A song needs that extra something to lift it from the ordinary.
Best of luck!