If you are hiring somebody to install a script for you, the first thing you should do is check their Trader Rating (for NamePros at least). Make sure they have more than just 1-2 positive reviews and hopefully have been here for more than a month or two. This will help determine if the person is trustworthy and can actually help you.
After that, most people simply give their cpanel username and password to the person they hired. The username and password are usually just PM'd or emailed (or some other method of private communication). The person who is hired shouldn't need anything else (other than maybe the link to log into cpanel if its not www.domain.com/cpanel, etc) because he/she should be able to upload/etc everything using cpanel.
Do this only after you tell the person you're hiring exactly what you want done, and he/she says that they're able to do it as well as you agree on a final price.
PS: If you need a script installed, send me a PM and we can talk.