Not too bad a job
turn off mains. undo clamp that holds door boot to cabinet and peel off the boot.
the seal is then held onto the tub either by a large circular spring or maybe a clamp with a screw.
if it is a spring, then just force the boot downwards and pull outwards and the boot will come off.
if it is a clamp, undo the clamp and pull the boot off.
look at the new boot and you will see a groove that slips onto the lip of the tub. lubricate this groove liberally with washing up fluid or similiar.
start at the top and push the groove onto the lip. work in both directions round the boot forcing the groove onto the lip until entirely in position.
if a clamp was fitted, just put the clamp in the outer groove and tighten
if it is a spring, this is more difficult and you will find an extra pair of hands helpfull. stretch the spring and push into position in the outer groove.
fit the boot to the cabinet and replace the clamp
test for leaks on rinse cycle as water level is higher. there will be some foam from the washing up fluid.
remember to make a note of which way up the door boot was fitted as there is a top and bottom