If you're talking about during the quest Demon Slayer, then what you do is use a bucket full of water with the drain, causing the key to dislodge and go into the sewers. Then go near the Church, and there should be a manhole. go down into the sewers, there should be two hallways. go through the hallway to the left, then turn left when you get to the water, and behind a broken wall, with (i believe) skulls piled up, there the key will be :) hope that helps!!!
bucket of water
Assuming that you used the bucket of water with the drain, the key should be here in the mud http://img502.imageshack.us/img502/6241/varrocksewers.jpg
just head down, into the sewer, now just follow the hallway north. Go to your left, and you will see a little glint where the key is, if you already don't see it.
Buy it, get it from the Hill giants.
use a rope on it and talk to that guy with you in the sewer
bucket of water
Assuming that you used the bucket of water with the drain, the key should be here in the mud http://img502.imageshack.us/img502/6241/varrocksewers.jpg
get a bucket of water pour it down the drain and get it in the sewer go to runehq.com or runescape.wikia.com for quest guides
in the drain in varrock place us water in the drainage pipe and then go to the swears to wear the pipe ends
just head down, into the sewer, now just follow the hallway north. Go to your left, and you will see a little glint where the key is, if you already don't see it.
use a bucket of water on the drain
sorry you cant make keys on runescape unless of corse it is the crystsal key.
Right click the water then select use then click the drain.
There isn't a key for it unfortunately.
A brass key.
It's Varrock Castle. To get it you need to get a bucket of water and use the bucket of water on the kitchen drain. Thie key will fall into the sewer, you'll need to go to the Varrock Sewers to pick up the key. The key will be on top of a pile of mud near a clump of several skeletons.
Buy it, get it from the Hill giants.