[slbmeh]

Just used this yesterday. Not the best method, but I think it's the easiest method unless you use a tool.

This really only works against sites that don't maintain sessions.

If you know the structure of the cookie write a script to recreate it with the values you want. Throw up a local webserver and host it there. Point the domain you are forging to your localhost in your hosts file. Navigate to the site the cookie is stored and you are ready to be on your merry way with a shiny new forged cookie.