To use a new font in a TEX document, select it specifically.
Example: in a TEX macro file, or in a LATEX style file,
to define \mine as a font-selection command for
`myfont10.tfm', say:
\font\mine=myfont10
Then to typeset `Mary had a little lamb,' in the myfont10 font, and then to revert to the previous font, type
{\mine Mary had a little lamb,}
Note, however, that this will not change the line spacing parameters
of TEX as
well. If your lines appear a little too cramped and unevenly spaced vertically,
it is very probable that you need to increase \baselineskip.
For LATEX users, a simple remedy is to just select a larger font
before your own. Also, end your paragraph by an empty line or a
\par command before the closing brace, or your line spacing changes
will be cancelled before the paragraph has a chance of being typeset.