Narbacular Drop Achievement
Place a portal under your co-op partner while they are gesturing
This can gained in either split-screen or over XBL.
To preform any gesture, simple press :udpad:and use the to to select what gesture you want to do.
This can be done as soon as you're heading to the first course. While you walk along the platform towards the door to the left you'll see a portal slab on the floor in the corner. Have your partner wait here over the slab.
Now head up and go through the door. Look to the right and you'll see another portal wall. Place a portal here and head back to your partner. Have them pull a 'wave' gesture and as soon as they do, put the opposite portal under their feet and the achievement will unlock.