Lift the door off the hinge pins and lay it on a table with the handle off the edge so it'll lay flat.
Clean out all the old glue with a screwdriver and a wire brush if necessary.
Wipe out the channel with rubbing alcohol on a rag and let dry.
Lay down a nice zig-zag bead of gasket cement all the way around the channel.
Peeling the paper strip off the gasket as you go, lay it into the cement peeled-side down.
Start laying the new gasket all the way at the bottom of the hinge-side channel, so the end touches the bottom of the channel, and run up the hinge side, across the top, down the hasp side and across the bottom, until you run into the side of the gasket at the starting end.
Cut the gasket a little long, and squish it into the side of the starting end and the cement at the same time, so you get a tight seam.
Hang and close the door immediately, and leave it closed at room temperature overnight.
Here's a tip from our gasket guru, Howard "Sweet Child" Lopeman: the reason your factory gasket lost its seal so quickly might be that the factory machine stretches the gasket a tad as it comes off the roll, making it a bit thin when it hits the channel. Make sure the new gasket is relaxed as you install it, not stretched out, paying special attention as you turn the corners, so it lays in as plump as can be. You'll get a better door seal longer.