I found a site once, but have lost it in a hard drive failure.  Run a google
search amd see what you
can locate.

If you can purchase the covers for the units ready to put on, that should be
easy.
If you intend to remove the old ones, sew new ones,  reinstall, this would
require attention
to selection of appropriate fabrics, pattern making, sewing, etc.

My mother learned how to do all this sort of thing herself, with no
instruction whatsoever.
With experience, it became easier for her.

You might be able to take off the old upholstery, take it to a shop which
has the industrial type
sewing machines and fabric choices, and have them sew up some new covers for
you.  Then
you could reinstall.  If the cushioning foam is damaged, you can replace it
at the same time by
simply cutting new pieces from foam stock.

   Only if the brush and wipe technique fails.  The high-pressure,
regulated to about that supplied by a do-it-yourself car-wash wand, would be
the most you should use.  I really have used the pressure technique more on
carpets than seats.  Most seats come out fine w/o use of pressure.  Be sure
to spray the stains directly with a cleaner like Purple Power first and let
them sit for 30-45 minutes.  Then the damp brush.  Then the damp towel.
Repeat process several times if necessary.  You should be amazed.  And this
can be done w/seats in place.
   If you have to wet them so much as in the pressure technique, yes, take
'em out, clean them using P/Power & detergent.  Bear in mind that the
detergent/cleaner along with pressure should do the work more so than
*drowning* them in water; and then let 'em dry in the sunshine for several
days.  This drying is important.  As they dry, you'll find you have to
re-wipe w/the towel repeatedly, as the stains will probably  rise--depending
on how deeply into the foam they reached.  But on the final wipe as they
completely dry, the stain will NOT rise again.
   Good luck and enjoy the comfort of cloth AND the (somewhat) cleanability
of vinyl.  s