To create a new DSSI plugin you simply create a subclass of DSSIPlugin and use the register_dssi() function to register it in your initialisation function. Details and an example can be found in the documentation for DSSIPlugin.
To create a new DSSI UI, just write the GUI program and use an instance of DSSIUIClient to handle the communication with the DSSI host. More details can be found in the documentation for DSSIUIClient.
There is also a small library for writing LADSPA plugins, using the class LADSPAPlugin.
You can get a tarball with the libraries here: http://ll-plugins.sourceforge.net/dsl/dssi-support-libs-0.5.35.tar.gz.