Hanging items on a nail isn't hard, but it's always tough to hang something you can't see. You need the string to thread the nail, but not the wooden frame itself. You could photocopy a template, but your poster or painting might be too big. A simple fork can make the job easier.
Place the fork on the nail, with the handle sticking upwards and the nail going through the middle tines. The handle will be at a slight angle, which is perfect. Thread the painting rear wire or string behind the fork handle, and slide it all the way down to the nail. Then just pull the fork up and out. Voila, you have a perfectly hung painting.
Use a fork to help hang a painting [Reddit]