Ok what i do is lean the washer backwards against a wall as this will allow the drum to drop back a little and gives a bit more room.
Then get a long screwdriver and push it through the hole in the door lock were the door pin goes.
Now hook the spring over the screwdriver and push it aganist the drum.
Now hold the screwdriver with you knee and then strech the spring clamp around the lip of the door seal and feed it all the way round.
As you get round the bottom it will start to get tight but try and get it round there and then pull the screw driver out from there.
The tricky part can be round the dryer vent at the top. What i do sometimes is remove this while is refit the seal.
Take the top off and you will see the dryer chamber on the top.
Towards the front of this remove the 7mm screw. Then remove the 7mm screw holding the front of it on the inside of the door and you can remove it from there.
Then refit the sping clamp as above and then refit the vent into place.
Try that and see what you find and let me know. These Zanussis have never been an easy washer to fit a door seal on.