1. make an enity misc_portal_camera
2. push n (add noswing) (targetname: cam1) (target: eye1)
3. add an enity info_notnull (targetname: eye1)
4. the camera will look trough cam1 in the direction of eye1
5. now u made your camera so now to add a tv to look at it :)
6. make a brush with one side (my_portal/portal_tex)
7. make an enity misc_portal_surface (target: cam1) and place it near the portal_tex texture
You're done :D
ps: don't add two portal_tex for 2 different camera close together else one will not work