To create a relationship, click the Relation icon as shown below and then drag the mouse cursor in the direction from the parent to the child table. 
Specifying the exact columns will create a relationship between them. Selecting table headings (without indicating columns) will result in the creation of new fields, between which a relationship will be created. Also note that the related columns should be of the same type. 
An example relationship is shown in the picture below: 
The name of the relationship, the columns that compose it and its other features can be edited in the property panel. 