Making this probably the shortest post on the site, HDLC is earth shatteringly simple to configure. By default it is already enabled on a Cisco router leaving you with nothing to do other than configuring the clock rate and the inevitable troubleshooting when you find your provider hasn't configured your circuit correctly. In the event that another form of PPP encapsulation has been configured on the interface, two words are all that is required to re enable HDLC.
First order of business is to ensure that HDLC is running on the interface. It is actually simpler to just enable it rather than bothering to check whether it is or not, which is achieved by executing the interface configuration command below.
The final step is to set the clock rate on the DCE end of the link. A number of clock rate speeds are available to you; to find which ones are supported by your interface, execute the first command below, followed by the second to set.
Router(config-if)#clock rate ? <- will display all clock rates supported Router(config-if)#clock rate <rate>