Tell yourself that you will write a song in the morning (when you are not busy.
Wake up and make your coffee. Having that, grab your notebook, a pencil with eraser.
Drink your coffee.
No TV or computer on.
Sit in silence.
I find that after 10 minutes or so some words come to me.
I write them down, but often suddenly I think of another theme and put brackets around those and write 3 or four lines for verse or chorus.
Do not touch your instrument. Start tapping out a rhythm on your notebook with the pencil. And try whispering the words, spoken and find a cadence in them.
Write (in parenthesis) the number of syllables next to each line of lyric and establish the general length of a line.
When 3 or four lines get their cadence, then pick up your guitar.
Your spouse, kids or house mates might be sleeping.
By keeping it all inside and down to a whisper, there is an excitement created and the rest of the song can be finished.
But be quiet.