Body lock passing when they are already flat

When I take someone down or sweep them, a lot of the time they are already flat on their back playing butterfly guard. I like body lock passing but its hard to lock up a body lock on people who are already flat and I have double unders. I usually just try locking up a grip under their head and slowly sliding it downwards. This takes a lot of time and effort though. Is there a better way to get the grip?