Redial
submitted this great tutorial to us which will show
you how to get those portals into your level
Portals
are actually fairly easy.
A portal consists of 3 to 4 objects. First you have
an object is a brush you want to be the
"viewer". Then you have 2 entities, the misc_portal_surface
and the misc_portal_camera.
This tutorial also uses an entity of target_position
which is used to aim your camera, although you can
give your misc_portal_camera an angle to aim
instead.
I will start with two rooms on my map.
One room is where the viewer is located, the other
has a cylinder and will be what we see in our
portal.

Create
an object which will be your "viewer", one
face of this object will be what the camera sees.

Once you
have your "viewer" brush, it must have a
portal_sfx shader (found in the sfx texture set)
applied to only one surface. The rest of the faces
you can apply pretty much any texture, it's
recommended to use either a clip or nodraw so when
your playing all you see is the portal and no box
behind it.

Create
an entity of misc_portal_surface and place it within
64 units of your "viewer" surface. Then
create a misc_portal_camera and place it in the
other room. In this example, I am also going to us a
target_position to tell the camera where to point.
It is also possible to just give the
misc_portal_camera an angle, but I find it easier to
point it at a target.

The
entities need to know who is supposed to do what, so
we need to give them targets and target names.
Starting with the misc_portal_surface, you need to
target it to the misc_portal camera. In qe4, the
easiest way to do this is to press Escape (ensures
nothing is selected), Shift-Click on your
misc_portal_surface, then Shift-Click on the
misc_portal_camera and press Control-K. Doing this
will give the misc_portal_surface a key of 'target'
that has a value of 't1' and will give the
misc_portal_camera a key of 'targetname' with a
value of 't1'. (qe4 will draw arrows showing what
targets what).

Repeat
the same thing for the misc_portal_camera and the
target_position. Press escape (make sure nothing is
selected). Shift-Click on the misc_portal_camera,
Shift-Click on the target_position. Press Control-K.
This gives the misc_portal_camera a key of 'target'
with a value of 't2' and gives the target_position a
key of 'targetname' with a value of 't2'.

That's
all there is too it. Add a info_player_start and
some lights, and you have a portal. Now read the teleport
tutorial to make it transport you to the
destination...

-redial-
Thanks Chris Brethen, Paul Jaquays, and Murphy
(c)