Define a new command; arguments are the command, the number of arguments, and the command body. For example:
\newcommand{\crazy}[2]{{\bf #1} is crazy but {\bf #2} is not.}
The command must have no whitespace at the start of the line or between the three arguments.
New commands may also be defined in the tex2rtf.ini file using slightly different syntax (see Macro not found errormacronotfound).