To get the yellow key on level 18, Spawned Fear:

Once you have the red key, go through the red-keyed door
and take the path that curves sharply around to your right.
Search the bookcases on your left to open up a hidden
passage.
Walk up the stairs to a big, open, irregularly shaped area.
There's one ground-level switch right around the corner to
the right.  Activate that to drop a platform, then Run to
get on the platform before it rises again.
When you're at the top you'll find another switch; Activate
it to drop another nearby platform.  Get on it and ride up.
Activate the switch you'll find there, and a final platform
will drop nearby; jump down and Run to get on it.
When you're at the top you'll see another switch.  Activate
this and the ground will shake to indicate a change in the
map.  Now jump down and follow the right-hand wall till you
come to another ground-level switch.  Activate that to
reveal a Soul Sphere nearby.
From there, walk ahead and look for a structure that, on
your map, looks vaguely like a backwards 7 or backwards
capital L, depending on how your map is oriented.  On one
end of this will be an open door.  This door lets you walk
up some steps and into a little chamber ahead where you can
see a switch.
Activate that switch to drop a nearby platform, and quickly
step back away from the switch and turn left, and jump down
to the platform before it rises up again.
Once you get to the platform and it rises, Run forward and
to the right, and you should see a switch close by in a
small cul-de-sac.
Activate this switch and turn around.  Nearby you'll see a
place you can jump down into.  This place is where a series
of tunnels come to an end, and the tunnel floors are lined
with corrosive slime, so be careful here...
Jump down in, and turn to face down the corridor.  Ahead of
you, some tunnels branch off to the left and right.
Two tunnels branch off to the right; take the left-most of
these 2 right-hand tunnels.  This leads into a short
dead-end passage, at the end of which is Radiation Shield
which you'll need here.  Grab it and walk back out, hugging
the right-hand wall.
Continue to carefully follow the right-hand wall around the
contours of this area until you come to some steps.  Walk
up steps and follow the path till you come to a gap in the
path ahead.
Back up to get a running start (hold down your Run button)
and Run across this gap.
Continue on till you come to another gap.  You should see
the yellow key up ahead of you.
Again, back up and get a running start to Run across the
gap and onto the raised area where the yellow key is.
You may have to attempt this jump more than once.  Using a
combination of Run-plus-Strafe works well (it makes you run
slightly faster, and therefore you can jump slightly
farther), but this can be tricky, as it moves your player
character at an odd angle.
(Check your Doom 64 manual and/or your controller setup in
the game's Options menu if you need more information on Run
and Strafe functions.)
Once you have the yellow key, hug the right-hand wall and
Run until you come back to your original starting point
(the ceiling overhead will be open to the sky).
Look left for a slightly yellow-tinted panel.  Activate it
like a switch, and this wall section drops, allowing you to
get on and ride back up, and out.
Done!