Getting Metafont to do what you want

Metafont allows you to create your own fonts, and most TeX users will never need to use it. Metafont, unlike TeX, requires some customisation: each output device for which you will be generating fonts needs a mode associated with it. Modes are defined using the mode_def convention described on page 94 of The Metafontbook (see TeX-related books). You will need a file, which conventionally called local.mf, containing all the mode_defs you will be using. If local.mf doesn't already exist, Karl Berry's collection of modes (modes.mf) is a good starting point (it can be used as a 'local.mf' without modification in a 'big enough' implementation of Metafont). Lists of settings for various output devices are also published periodically in TUGboat (see TUG). Now create a plain base file using inimf, plain.mf, and local.mf: 

   % inimf
   This is METAFONT...
   **plain # you type plain
   (output)
   *input local # you type this
   (output)
   *dump # you type this
   Beginning to dump on file plain...
   (output)
This will create a base file named plain.base (or something similar; for example, it will be PLAIN.BAS on MSDOS systems) which should be moved to the directory containing the base files on your system (note that some systems have two or more such directories, one for each 'size' of Metafont used).

Now you need to make sure Metafont loads this new base when it starts up. If Metafont loads the plain base by default on your system, then you're ready to go. Under Unix (using the default web2c distribution ) this does indeed happen, but we could for instance define a command mf which executes virmf &plain loading the plain base file.

The usual way to create a font with plain Metafont is to start it with the line 

\mode=<mode name>; mag=<magnification>; input <font file name>
in response to the '**' prompt or on the Metafont command line. (If <mode name> is unknown or omitted, the mode defaults to 'proof' and Metafont will produce an output file called <font file name>.2602gf) The <magnification> is a floating point number or 'magstep' (magsteps are defined in The Metafontbook and The TeXbook). If mag=<magnification> is omitted, then the default is 1 (magstep 0). For example, to generate cmr10 at 12pt for an epson printer you would type 
  mf \mode=epson; mag=magstep 1; input cmr10
Note that under Unix the \ and ; characters must usually be quoted or escaped, so this would typically look something like 
  mf '\mode=epson; mag=magstep 1; input cmr10'

If you don't have inimf or need a special mode that isn't in the base, you can put its commands in a file (e.g., ln03.mf) and invoke it on the fly with the \smode command. For example, to create cmr10.300gf for an LN03 printer, using the file 

   % This is ln03.mf as of 1990/02/27
   % mode_def courtesy of John Sauter
   proofing:=0;
   fontmaking:=1;
   tracingtitles:=0;
   pixels_per_inch:=300;
   blacker:=0.65;
   fillin:=-0.1;
   o_correction:=.5;
(note the absence of the mode_def and enddef commands), you would type 
   mf \smode="ln03"; input cmr10
This technique isn't one you should regularly use, but it may prove useful if you acquire a new printer and want to experiment with parameters, or for some other reason are regularly editing the parameters you're using. Once you've settled on an appropriate set of parameters, you should use them to rebuild the base file that you use.

Other sources of help are mentioned in Metafont and MetaPost Tutorials.

modes.mf
fonts/modes/modes.mf

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=useMF