This isn’t a step by step guide, more a set of pointers.
First, you’ll need to remove the door card. Easy enough, just a few screws and a few poppers. Make sure before you start that you have either a new plastic door seal sheet, or something suitable for patching up your old one as you’ll need to get behind it.
Taking off the old seal isn’t to much of a problem, it’s fairly obvious, although you’ll be left with a whole bunch of little plastic clips in the door which will need removing. If there’s an easy way to do this, I don’t know it, so as far as I’m concerned this involves a flat head screwdriver, a pair of pliers, and a whole bunch of swearing. When you come to do the ones on the bottom of the door, put something soft on the floor.
Attaching the new door seal is fairly straightforward, except for one part. The seal runs around the bottom of the door, up over the quarter window, and down the back of the quarter window where is disappears into the door. This is where it gets a bit tricky. You could probably do this with relative ease if you removed the window but being lazy I didn’t bother. To start with, wind the window down completely. Slide the weather seal down as far as you can in the little channel. Once you’ve done this, wind the window back up about 3/4s of the way, then feel in behind the door panel where the seal is poking out. Grab this and pull down from the bottom, which should guide the seal into place. The bit you pull down, it’s best if you let it come out of the channel while you’re pulling it as it’s easy enough to push back into place afterwards. It took me a while to figure this out, as I was trying to push the seal in from the top and it was bunching up quite a bit. I wasted my time, but you don’t have to. Then all you have to do is pop the little screw back in at the top, pop the poppers in, and reattach the door card.