At what appears to be the end of level 19, you will be on a platform heading down into a furnace. Before you hit the flames, you must shoot a portal across from you on the safe ledge, and next to you. Once you are up on the ledge, you will notice another dirty ledge across from you. Shoot a portal high on the wall facing the dirty ledge and a portal on the wall next to you. walk through this portal, and as you fall out of the higher portal shoot one at the ground below you so that you will jump across the gap to the dirty ledge. The achievement should then unlock.