reaplacing door seal for Siemens washing machine.

Assuming it's like every other washing machine I've met, it's dead easy.

Open the door and look around the outside edge of the seal until you find the retaining device - a wire, usually, with a retaining screw, though other systems exist. Undo it. Push the seal into the drum and find how the inner join is made. Undo it. Remove the old seal.

Reassembly is the reverse of disassembly. Just make sure that any "features" are correctly oriented - things like drain holes and gush deflectors.