This article should definitely help with that. AllScan - How To - Build a High-Quality Full-Duplex AllStar Node for Under $150 No COS input is needed if you use ASL’s usbradio driver, but if you’re connecting repeaters then I would recommend modifying the HT to bring out a hardware COS line.
You would want to configure the digirig to use the CM108 PTT output instead of the serial port RTS line. See COS Connector Use post for details on that.