Unplug the machine from the electrical supply. Open the top of the washing machine, usually by a couple of small screws on the sides or rear.
If the leak is small and the machine only has another year to go, then consider what a friend did. He turned the seal so the leak was at the top, then used his kids cycle puncture repair outfit.
To replace the whole seal, make a simple mark on the rubber and the drum, so you know which way it fitted. Locate the outer sealing edge of the rubber seal. There may be a clamping strap which keeps the rubber seal in position over the drum. Unclip any clamp and slide off the rubber seal. Take particular notice how the inner seal fits inside the static part of the drum area. Inspect and decide if you want to rotate the seal so the leak is positioned to the top, hoping for a temporary repair. This also works if the door seal is leaking, with the leaky area repositioned at the top. This is useful as an emergency until a new replacement seal is available, especially if needing to do the washing on a daily basis.
The new seal is checked for position and the way it fits over both the inner and outer sealing ring areas. Take your time to coax it into position, and do not skimp checking that it has been fitted properly all the way around, especially at the bottom where it is harder to reach. Take your time and do it well. If necessary, lean the machine to one side and check from underneath, as this is where the water will try to find it's way out.
Cost is a bicycle puncture repair patch if skint, or a new seal for about a tenner.
Tools: Phillips screwdriver and possibly a pair of pliers.
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