Log out of Apple ID and Log in with new Account

Hi

I have a MacBook Pro that I am selling to a new owner. I have wiped the Mac clean of all my data, formatted it and reloaded it with El Capitan. The OS X asked for an Apple ID when I was setting it up, so I set up a dummy Apple ID so that I could show the Mac working. I only used it so get into the Mac - I did not log into iCloud or iTunes.

I've now got a buyer for it an I need to log out of my dummy Apple ID and they need to log in with their ID but I cannot find how to do this.

Can anyone please list the steps I need to take to log out and log in to a different Apple ID on the MacBook Pro?

Thanks

Richard