Associated Press “Robert Plant recalls to court the night Led Zeppelin made Stairway to Heaven” https://www.theguardian.com/music/2016/jun/21/led-zeppelin-stairway-to-heaven-lawsuit-robert-plant
Plant had a much sharper memory of creating Stairway at Headley Grange in England, where he said his goal was to evoke an image of pastoral Britain.
Plant said he was sitting by a fire in the building when Page played the introduction to the song. Plant said he had a couplet that might work that began: “There’s a lady who’s sure all that glitters is gold and she’s buying a stairway to heaven.”
From there it started “rolling pretty fast”, he said.
The other band members would make contributions and Plant would occasionally retreat to a bedroom with a notepad to work on more lyrics for what became an eight-minute song.