So ...

Upon activation, go to Appearance => Widgets and simply drag the newly added "Feature A Page Widget" into your sidebar widget area.

First of all, you'll have to choose a title ... Something that resonates with the goal or the content of the featured page.

Next, you need to select the targeted page from the "Page to Feature" drop-down list. As soon as you select a given page, the widget will show you the actual page status checking the thumbnail and the excerpt. If the featured image and/or excerpt isn't (aren't) avalaible, you can click the small pencil icon to open the page editor where you can set a featured image and/or excerpt.

Finally, you can choose a layout (wrapped image, banner image, big image) and the page elements that will be displayed in the sidebar (title, excerpt, featured image, "Read More" link):

Note: if you can't find the "Excerpt" meta box under your page editor, then select "Screen Options" on the top right corner of the screen, and check the "Excerpt" checkbox to enable it.

OK. Now let's see the result ...