I'll drive you, but in exchange, you'll create a wiki page for that after testing it, good deal, no ? :)
Actually it's the same as if there were a simple one-to-many, many-to-one links.
Code:
<class name="Employer" table="employers">
<id name="id">
<generator class="sequence">
<param name="sequence">employer_id_seq</param>
</generator>
</id>
<property name="name"/>
<set name="employments" lazy="true">
<key>
<column name="employer_id" not-null="true"/>
</key>
<one-to-many class="Employment"/>
</set>
</class>
Same for the other side.
For Customer/Order/Product, you can't do it using a composite-element, composite-element are only related to it's father. So you need to replace composite-element by an Entity.