I am assuming you have the cartridge type filter, not a spin-on conversion....
There's a pressure ring at the bottom of the canister that sits around the central post. It goes against the bottom of the paper filter. this can fall out easily, so if you don't have it, it could be lost and you need it. At the top, there is another pressure seal that's spring loaded. You should be able to push it up and down up inside the housing (that's mounted to the block). The spring is pretty stiff, so use some muscle to convince yourself its there.
Next, there is a large square-cut O-ring that seals the canister to the housing. Probably should be replaced at each change.
I have had trouble finding the square cut rubber washer that fits over the bolt, at least one that's for oil service (plumbing parts are exact size, but for water service). A round cut O-ring may suffice.
My oil housing/canister assembly is all off the car at the moment, so if need be, I can send a pic of what all this stuff looks like.
Cheers,
Jeff