Download your preferred Xopus release (i.e. Xopus-2.1.79) from http://xopus.com.
Follow the installation instructions to install it on your server (in this case we are unzipping to the Tomcat webapps/ROOT directory).

Configure the Xopus context and path in (local.)build.properties, e.g.

xopus.context=Xopus-2.1.79
xopus.path=relative/path/to/tomcat/webapps/ROOT

Set these same two parameters in modules/xopus/config/cocoon-xconf/input-modules/xopus.xconf.
[optional] If you wish to use a resource type other than xhtml, create a resource type-specific XSLT file in $PUB_HOME/modules/xopus/xslt/{resource-type}.xsl.
[optional] This applies only if you do step 5. Create a resource type-specific XSD schema file in $PUB_HOME/modules/xopus/resources/schemas/{resource-type}.xsd.

TODO: Add the xopus module call to menu.xsp

(Currently menu.xsp only recognizes usecases when creating the URL, we need to modify the stylesheet to accept lenya.module in addition to lenya.usecase. It should look similar to what is shown below.)

Note by Andreas Hartmann: I don't think we should use a module URL but rather a usecase URL.

menu.xsp:

[...]
<menu i18n:attr="name" name="Edit">
  <xsp:logic>
    String doctype = <input:get-attribute module="page-envelope" as="string" name="document-type"/>;
    if ("xhtml".equals(doctype)) {
    <block info="false">
      <item wf:event="edit" uc:usecase="bxe.edit" href="?"><i18n:text>With BXE</i18n:text></item>
      <item wf:event="edit" uc:usecase="editors.oneform" href="?"><i18n:text>With one Form</i18n:text></item>
      <item wf:event="edit" mod:module="xopus" href="?lenya.step=open"><i18n:text>With Xopus</i18n:text></item>
    </block>
    }
  </xsp:logic>
</menu>
[...]

Build Lenya.
Try to edit your document by adding ?lenya.module=xopus&lenya.step=open to the url and hitting enter. This will eventually be replaced by the menu entry in step 7.