First, stand in the crotch between the barrel and the plywood.
Position your crosshair right above this portion of the roof. Prime your nade and then press your JUMP THROW bind.
This is where the molly will land:
Banana to Quad Molly
A cheeky molly. Hard to get right, but when you do it is very useful for cutting off hiding spots on B site. Equally useful on the retake.