There are two ways to create constraints: by editing the start or finish date of the task, or by editing the Task Information dialog. When editing a date, the constraint you get from the edit depends on which date you edited (Start or Finish) and how your project is scheduled (from the start date or from the finish date).
Editing a Date
Select the date you want to edit for the task you want to constrain.
Enter a new date.
The constraint will change or be added for the task depending on which date you edited and how the project is scheduled.
If you edit this task date
and your project is scheduled from this date
then this constraint will be created
Editing the Task Information Dialog
Double-click the task whose constraint you want to edit.
Click on the Advanced tab.
In the Constraint Type drop-down, select the type you want.
For all types EXCEPT ASAP or ALAP, enter a date for the constraint.