first, get on the map 'Highwinds', its by far the easiest. Next, put a portal on the wall in the sky in the back corner, and then place your other portal in convenient spots and hope someone goes through it, this is a very situational achievement, which can be annoying. Refer to the video below for better clarification of where to put the portal. Again, this is entirely luck based, but this is the easiest/quickest method