Your History Palette won't look just like this, but the important thing is that your last History State will be the Gaussian Blur, which you just applied in the step above.

Select the previous layer (blue in this example), and click the box to the left of Guassian Blur, which will place the History Brush icon in that box. This means that when you paint with the history brush, it will be painting NOT with color, but with that History state. Confusing, mabye, but you'll see how it works in a minute.

Your History Palette won't look just like this, but the important thing is that your last History State will be the Gaussian Blur, which you just applied in the step above.

Select the previous layer (blue in this example), and click the box to the left of Guassian Blur, which will place the History Brush icon in that box. This means that when you paint with the history brush, it will be painting NOT with color, but with that History state. Confusing, mabye, but you'll see how it works in a minute.