how to write finder-query and methods

i'm working on EJB.as you know the deployment descriptors were written in XML in EJB.
i have problem in writing finder queries in weblogic.cmp.rdbms-jar.xml file.

for example i finder query/method to get records greater than 5000 is....

<finder>
<method-name>findBigAccounts</method-name>
<method-params>
<method-param>double</method-param>
</method-params>
<finder-query><![CDATA[(> balance $0)]]></finder-query>
</finder>

i want some examples query like this to to understand them.
for example if i want to perform the following tasks what should i do.

1. get the meximum user_id.
2. get the last-names that are not NULL.