See the OJS 2.4.x Crossref Plugin Guide.
Setup for the Crossref Export Plugin in these versions is the same as detailed in the OJS 3.1.2 Crossref Plugin Guide
Export XML will download XML generated by the plugin in the Crossref XML Schema. Simply click the check boxes next to the articles you want to deposit and then export. Once you have the XML file, you can submit it via the Crossref admin tool at https://doi.crossref.org.
You can read more about the Crossref admin tool at https://www.crossref.org/documentation/member-setup/direct-deposit-xml/admin-tool/.