Anna, It was Photoshop Advanced#3, lesson 4a. I keep a text file of all the lesson topics, so I can quickly go to that to see where I learned something. Andrea's method in this lesson is a little different from the technique Lisa mentioned. Always different ways to accomplish something in PS.
BTW, great blog! It's a super way to share the LOs you're doing with family, etc. Link it up to your FB page, too.
I've got a new blog, too: This 'n' That by Donakat