Popup Box With Cookie (Yes Or Cancel Prompt)

I'm trying to make a code for something on a forum. It will prompt the user for a yes or cancel, then save a cookie with their answer. Then the next time the script is called, it will check for the cookie. If there is one saved, it won't prompt and use that value.
Here's my breakdown (hope this is right)

1. Have a popup box that prompt for a 'yes' or 'cancel'
if(window.confirm("Use WHATEVER theme?"))
{
* * location.href = "URL HERE";
}
2. Save a cookie with the answer. I really don't have a clue other than the fact that you use setCookie(name, value, expire). I'm guessing that there will need to be something special in the value section.
3. When the script is loaded, it will check for the cookie saved and do whatever was save. Again, I know I'll need the getCookie function, but I don't know what else.