First, you have to find emc-cmis.war.
Once you have it, put it in the web application server that already contains your dctm-rest.war (present by default), and put emc-cmis.war besides it.
Tomcat (for instance) will automatically extract it into an emc-cmis folder besides the WAR. In my limited experience I have seen a case where the emc-cmis folder contained another emc-cmis folder that contains WEB-INF/etc, in such a case remove the extra level.
Within the emc-cmis folder, there should be a dfc.properties that you will need to modify, here is an example:
dfc.data.dir=/home/dmadmin/dctm/product/7.1/dfc_shared
dfc.tokenstorage.dir=/home/dmadmin/dctm/product/7.1/dfc_shared/apptoken
dfc.tokenstorage.enable=false
dfc.docbroker.host[0]=demo-server
dfc.docbroker.port[0]=1489
dfc.session.secure_connect_default=try_native_first
dfc.globalregistry.repository=MyRepo
dfc.globalregistry.username=dm_bof_registry
dfc.globalregistry.password=password
Any Documentum server contains a lot of dfc.properties that you can get inspiration from.
Then restart Tomcat, or the whole Documentum, and open http://localhost:8080/emc-cmis/main , this page should show up:

The URLs are:
More information: https://www.emc.com/collateral/TechnicalDocument/docu57844.pdf