How to Add a Recent Items Folder to Your Mac's Dock

1. Open a Finder window and select File -> New Smart Folder in the menu bar. Alternatively, right-click (or Ctrl-click) the Finder icon in your Dock and select New Smart Folder.
2. In the Finder window that opens, make sure the search header is selected as This Mac, then click the plus icon in the upper right of the viewing area.
3. Select Last opened date in the first search criteria dropdown.
4. Select within last in the second dropdown.
5. In the third and final dropdown, select how far back you'd like the folder to show recently opened items for. Your options are days, weeks, months, and years.
6. In the input field to the left of your timescale selection, specify the number of days/weeks/months/years of recently opened files to show.
7. To restrict your recent items folder to a certain kind of file – images, for example – click the plus button to add another row, then select a kind from the second dropdown. (Depending on the kind you select, you may be offered an additional dropdown to further limit the criteria.)
8. Now, hold down the Option key. Note how the plus icon at the end of the row changes to an ellipsis. Click this, and you'll be able to add search parameters that exclude certain items like applications and folders from your recents list.
9. In the second row dropdown, select None of the following are true.
10. In the third row, first dropdown, select Kind.
11. In the third row, second dropdown, select the type of item you wish to exclude.
12. If you want to add more exclusion criteria, hold the Option key and click the ellipsis icon in the first row. When you're done, click Save in the upper right of the viewing area.
13. In the save dialog that appears, give your smart folder a name, and select your Desktop as the location for your folder. You can also opt to include the folder in Finder's sidebar by checking the box next to Add to Sidebar.
14. Click Save.
15. Switch to your Desktop and drag and drop your new smart folder to the right of the the Dock, which will automatically move any existing icons behind the divider to make space for it. (If you like, you can delete the same folder on your Desktop once it's safely docked.)
16. Finally, right-click (or Ctrl-click) the docked smart folder and select Folder in the contextual menu.