I figured out how to do it. The following lines need to be added to the build.xml, under the tab Files of the corresponding project on NetBeans:
<target name="-post-jar" depends="Signing Procedure">
</target>
<target name="Signing procedure" depends="">
<echo message="Signing ${dist.dir}/MyAppet.jar..."/>
<exec dir="${dist.dir}" executable="C:/Program Files/Java/jdk1.7.0_25/bin/jarsigner.exe">
<arg value="-verbose" />
<arg value="-keystore" />
<arg value="C:/Program Files/Java/jdk1.7.0_25/bin/MyKeyStore.jks" />
<arg value="-storepass" />
<arg value="mystorepassword" />
<arg value="-keypass" />
<arg value="mykeypassword" />
<arg value="C:/Users/Charles/Documents/MyNetBeansProjects/MyProject/dist/MyApplet.jar" />
<arg value="MySelfSignatureAlias" />
</exec>
</target>
I hope this is useful to other users!