Enter dependency relationships
This Document can be found in the following categories:
Dependency Link Toolbar Button
Multi select a group of tasks that have a finish to start
relationship and click the Link toolbar icon.

An alternative is to use either or both of the Predecessor and
Successor fields.
Direct Entry in Predecessor or Successor
Fields
Enter the row number of one or more predecessors in a
predecessor cell. The row number of the target will appear in the
list of successors for each predecessor.
Note: Entering predecessor-successor
relationships does NOT automatically reschedule task dates. You
must use either the Reschedule task dates or Compress schedule
command to calculate dates based on dependency
relationships.
If these fields are not visible, right click in or to the right of
the title row and select Insert Columns. Then select the desired
field from the field selection dialog. 
Note: A sub-task cannot have as a
predecessor any summary in its branch of summaries.