If you're making a portable you probably need something to watch it on. (Unless you want to guess what's happening in the game, but I wouldn't advise that) Anyway, this forum is your "Hacking a pocket TV/screen" one-stop solution. Share your experiences and knowledge here.
You gotta hook up two wires: the ground and the video wire. You're connecting the video wire in the right spot: you just need to run a wire from a ground on your NES to any ground spot on the PSone screen.