To find the keys in Doom level E1M6, Central Processing:

When you first appear on this level, you'll be in a wide
recatangular room near the middle of the south side of the
map, and you'll be facing north.

Step forward/north a few paces, and you'll be standing at a
4-way intersection: There is a closed door to the north, a
concrete walkway leading east across a slime pool, another
walkway through slime leading west, and the starting room
behind you to the south.

This 4-way intersection will be the starting point for the
instructions below.

RED KEY
-------
At the 4-way intersection, turn to face east, and follow
the concrete walkway to a large open room.  As you walk
into this room, the doorway you entered through will close
behind you, and remain closed for a time.
Look for the red keycard, which will be on the floor near
the far east wall of this room.  Once you have the key, and
the doorway has re-opened in the west wall, head west back
to the 4-way intersection.

BLUE KEY
--------
(To acquire the blue key, you'll first need the red key.)
At the 4-way intersection, turn to face west, and follow
the concrete walkway until you come to a pair of red-keyed
doors.  Go through the right-hand door, and follow the path
until you come to an opening on your right.
Walk southwest a few paces and you'll see the blue keycard
on the ground at the edge of a dropoff above green slime.
Grab the blue key, and go back out through the red-keyed
door, and head east, back to the 4-way intersection.

YELLOW KEY
----------
(To acquire the yellow key, you'll first need the red and
blue keys.)
At the 4-way intersection, turn to face north, and open the
closed door ahead of you.  Go through, and head towards the
right, and out the first open doorway on your right.
Proceed east, along a straight walkway across slime, and go
through the blue-keyed door at the far end.
Walk to the middle of the room and turn right.  Head south
and follow the path around to the middle of the next room.
(To your right, in the west wall of this room, you'll see a
wide vertical band of dark material between a pair of metal
strips.  This is a door that will be opened by activating a
distant switch.)
Continue south, and follow the winding path around, south
and east, to the middle of the next room.
Turn right and head south.  Follow the passage a few paces
until you come to a switch on the wall to your right.
Activate this switch, then turn around and reverse your
steps, heading west and north, back to the room with the
now-open doorway in the west wall.
Go through this new door and follow the passage west, and
up a few steps to where you'll find the yellow key on the
floor, next to a barred window.