This sounded like the "next step" in adding source citations to the FamilySearch Family Tree, so I wondered how it worked. The FamilySearch blog post describes the process, and the comments highlight some of the problems encountered so far.
Here is my look at it:
1) I signed into FamilySearch.org with my FREE account (that is a necessity to use My Source Box or the FamilySearch Family Tree). Then I searched the FamilySearch Historical Collections for Isaac Seaver, born in 1823-1824 (I used an "exact" search) and the results page appeared:
2) I clicked on the top search result for Isaac Seaver in the 1900 Census and the result summary page appeared:
Up in the right hand corner of the screen above is a link to "My Source Box" and running your mouse over it shows two morel inks - "Add to My Source Box" and "Go to My Source Box."
3) I clicked on the "Add to My Source Box" link and a popup window appeared:
The popup window is headed "Added to My Source Box" and shows the source text that will be added. I clicked on the "OK" button and the source was added. The source text for this record is:
"United States Census, 1900," Isaac Seaver, Leominster town (southeast quarter), Worcester, Massachusetts
4) I went through the list of search result matches and added sources for seven results to My Source Box. Then I clicked on the "Go to My Source Box" link and saw all of my sources that I had added:
5) I clicked on several of those on the list and was able to see more information for these sources - including the URL and the complete citation: