Most of the features that you will want to customize can be set with resources in the .Xdefaults file in your home directory. However, key bindings, mouse button bindings, and menu definitions for mwm are specified in the supplementary .mwmrc file, which is referenced by resources in the .Xdefaults file.
If you do not have a .mwmwc file in your home directory, you can copy it as follows:
+--------------------------------------------------------------------------------+ |% cp /usr/lib/X11/system.mwmrc ~/.mwmrc | +--------------------------------------------------------------------------------+
If you do not have a .Xdefaults file in your home directory, you can create one with any text editor. Once you have these files in your home directory, you can set resource values in them as you wish. Because you've made your own copy of the the .mwmrc file, your specifications will not interfere with the specifications of other users.