Detailed information on how to make a new or existing installation portable has been moved to the OpenMPT Manual (https://wiki.openmpt.org/Manual:_System_Setup#Portability).
Older information for OpenMPT versions 1.18 to 1.28 (https://wiki.openmpt.org/index.php?title=Manual:_System_Setup&oldid=3566#Portability)