Before you even think about configuring the font declarations by
producing a file fontmath.cfg or fonttext.cfg, you should
read the documented file fontdef.dtx. This is the source file from
which the default files fonttext.ltx and fontmath.ltx are
produced; it contains information concerning the
contents of the default files and what sort of customisation is
possible. In particular, it describes in detail the effects of
individual customisations on document portability including: which
customisations can be made without endangering the ability to exchange
documents with other sites (even if the formatting differs); and which
things should be left untouched because they will make your system so
different from others that the documents it produces will be
non-portable.