Putting the new upholstery on is not hard, but take the time to do a
complete job.  Strip the seats of their old skins carefully using a
sharp razor knife, but don't cut into the pads if they are still in
good shape (not very likely, but you might get lucky.  Look for any
metal rods that are pushed into the edges of the old covers, you will
need to reuse them in the new covers.  Also, separate the backs from
the bottoms -- I know that seems obvious, but some people don't and
pay for it with a difficult and crappy job.  Check the seat frames and
springs for broken pieces and repair anything you can, including rusty
sections (seal with epoxy paint after wire brushing as much as you can
off.  My first job went smooth, looked good, but went to hell quick
because I didn't take the time to prep to frames before putting on new
foam pads and upholstery.  Anything sharp will work its way through
the covers eventually, and sooner than you think.  My biggest mistake
was not shelling out about $10 for some heavy canvas (sissel) to
stretch over the bare frames before putting on the pads.  Just
hog-ring the canvas to the frames so that it has some room to move
with the springs when sat on (you can even sew a few loops through the
canvas in the middle of the seat bottoms and backs to be sure).  And
some tips:

- make sure you don't break off any of those short points attached to
the frame; bend them up carefully with needlenose pliers, and use
something hard and flat to press them back down after the upholstery
is on.

- make sure you work with warmed-up vinyl (if that's what you have). I
put mine out in the summer sun for awhile, but I think you can use a
short spin in the clothes dryer if you turn them inside out first.

- great trick I used on the seat backs:  cover over the padding with a
kitchen trash bag, just "bag" the seat back, pull the upholstery down
over the back with ease, then reach up and tear the bag out from the
inside -- quick, easy and smooth....

- find a wood box or something similar to press the seat bottoms down
on (upside-down of course) to compress the springs and padding before
you secure the covers.

The rest I think you can figure out, just remember to take your time
in preparation for the covers, but know you must work quick to get the
covers in place while they are warm and pliable.  If you have trouble
with one, don't force the issue, just pull it off, warm it up again
and give it another shot.

I can't think of anything else critical...good luck