To link a source to a fact, select the fact in the Facts and Events table so its details appear below. Then either click the + button below the Citations label, or right-click the fact and choose "Add source citation".
A dialog appears to select the source. By default, the most recently edited source is selected. Click OK to create the citation. The source then appears in the Citations list, identified by its record number (e.g. [S2]).
Citations can be removed using the trash button, and additional options are available by right-clicking the citations list.
See also Local citations for a lightweight alternative.