From Manila: 

 

There are several bus companies in Cubao (easiest being in the Ali Mall terminal) that are bound for Matnog or its vicinity. The trip takes 11 to 16 hours, depending on type of bus or if it's during the day or night. Some companies to consider are: Penafrancia, Philtranco, or Raymond Bus.

 

From Legazpi:

 

From the Legazpi bus terminal there are busses bound for Sorsogon on a regular basis. Make sure to get on one that is going to Bulan and ask the driver to stop at small town by the name of Trece. From Trece it is a short jeepney ride to Matnog.