You define instance of objects using the object keywords.
object "My Second Object" as o2
Relations between objects
Relations between objects are defined using the following symbols :
It is possible to replace -- by .. to have a dotted line. Knowing those rules, it is possible to draw the following drawings. It is possible a add a label on the relation, using : followed by the text of the label. For cardinality, you can use double-quotes "" on each side of the relation.