Bring a friend along, and ask him to be downed on a pallet without dropping it. Keep crouching nearby so the killer doesn't notice you (don't run, otherwise you'll generate scratch marks which the killer can see, so he'll ignore the pickup to look around) . Make note of which side of the pallet the killer is coming from, cause he may be able to body-block the pallet drop by starting the pickup right outside the region where you'll be able to drop the pallet (so it's better to ask your friend to move a bit to one side or the center so that you'll be able to get the drop prompt without an issue)
Once the killer starts picking your friend/teammate, run to the side of the pallet which you'll be able to drop and wait! Don't drop the pallet immediately, the killer won't be stunned mid-animation, and the dropped pallet will be wasted. The pick-up animation ends right after the killer has completely put the survivor on his/her shoulder. Only when he's able to move can you stun him with the pallet, so keep your timing ready and once you notice the killer's face change direction or move even the slightest, drop the pallet to stun the killer.
Now you need to do this 4 more times, but it's highly unlikely you'll be able to do it twice in the same match, as the killer will be more cautious after it has happened one time. Bringing the perk 'Windows of Opportunity' may help you see the auras of pallets if you don't have direct line of sight from the place you're hiding. Bringing the perk 'Sprint Burst' may be useful in reaching the pallet faster so that you have more time to observe the animation calmly to get your timing right.
P.S: It doesn't matter if the killer faces the direction of the pallet you're coming from, you don't need to drop from the back cause he can hear footsteps from either direction, so he can't do anything about it if your timing is right