Step 1# Place a portal down in a secure location. this will be portal 1

Step 2# using the 45 degree slope build a ramp starting at the base of the teleporter. You will need to go up about 2 or three roof pieces

Step 3# add a platform at the top of the slope. On this platform you will place another portal with the exit facing the slope. This will also be portal 1

Step 4# Build a platform above head height over the ramp. Place another portal on the edge of the platform facing out. This will be portal 2

Step 5 Wait until your friend is AFK then build the other end of portal 2 and push them through.

If all goes well you will have created a endless portal slide. They will slide down into the portal at the bottom only to be teleported to then slide down again. When your friend comes back they will be confused. You can trap someone this way with some walls.