At minimum you will need the buyer's name, address, and PAL number. As well as the information on the registration certificate such as registration number, make, model, caliber. Then you call the Canadian Firearms Center to initiate the transfer into the buyer's name. You will be given a transfer reference number, write it down and give it to the buyer so they can call in and confirm the transfer. Then you wait, maybe an hour, maybe a month, maybe longer.
The CFO will process the transfer and notify you when it is complete, however if you do not specify otherwise they will send you a letter, which might add a week to your wait time. If you ask, they can send you an email instead.
Then, if you want to mail the firearm you will need an Authorization to Transport to the post office/courier depot. If you will not transport the firearm, such as if you arrange door to door service, you will not need an ATT for that. If the buyer wants to come to you and pick up the gun, or to pick it up at the post office, he will need an ATT to take it home.
Then you'll need to get it packed up and ready to ship. No ammo can be sent through Canada Post, none at all and you'll get in trouble if they find any in your package. Put some sort of lock on it, or a ziptie through the action/on the trigger will work as well. Then please get a suitable box for it, or make one. It's not too hard to put together a frankenbox. Just make sure nothing is bouncing around, and no parts are rubbing together, and it should arrive okay. If you can, put cardboard or something over the muzzle, so it doesn't stab through the box. If you shake the box and things rattle, thet's bad, because CP might open the package to see if something broke.
Regardless of what type of box/case you use to ship with, having some sort of wrapping paper is good as well, especially if you're using a gun box with logos and words on it. I prefer to put about four dollars of tape on the wrapping paper, to make sure it's not going to tear and protect the label.