Paul wrote the lyrics except for the "horror grips us as we watch you die"-verse, a contribution by Stephen Stills.
Part of the lyrics, "take a sister", Paul took from the very first song "Fly Away", that he wrote when he was in college.
Another part, "If you smile at me", was modified from a message Crosby found at the side of a Baptist Church.
All this according to Tamarkin "Got A Revolution". Buy it, though it is a bit disappointing, only 200 pages concern JA. But together with the liner notes he wrote for several re-releases of JA albums, it will answer all your questions.