And here are the instructions to install the game via PortMaster. Bottom line: once you have the PortMaster files installed for Undertale, move the Linux game files into the ports/undertale/assets folder.
if you bought the game on gog then just go to the undertale root folder you know the one where all the files are located like data.win if you dont know where your undertale folder is then i cant help you since i bought undertale on steam