That is a youkai, get the most ancient sharp thing in the house and wedge it between the first and second cervical vertebrae of the youkai, now move it a bit up and down as if you were using a lever, this should sever the connection between the brain and the rest of the body, leave the object there, if it's ancient enough the youkai should be already dead but we want to be sure.
Now get another sharp thing and start cutting the chest in a "Y" shape centered on the sternum, keep the flaps of flesh separated.
Now get a saw and a thick rod for leverage, you want to remove the ribs between the top and halfway point of the ribcage, this will allow you to access the heart.
Now remove the heart, light a fire and burn it, keep the fire burning, you will need it later.
Grab a heavy blunt object and smash the head, you might want to use a crowbar or something capable of wedging into the skull in order to properly open the cranium, now with a knife start removing the brain, the cerebellum and the medulla (a "rope" of fibers going toward the vertebral column), burn those pieces too.
Now burn the body adding wood as needed, wait until the body is reduced to a fine ash.
Once the body is completely burnt, you can remove the sharp object you lodged into the spinal column.
Gather the ashes into a ceramic pot, have the pot sealed by the nearest priest, he'll take it from there.