Removal of the springs.
Using a set of spring compressors depress the spring, then place the strut in a vice. You will need to hold the damper in position with a T40 socket whilst you undo the top nut with an 18mm ring spanner or open socket.
Remove the rubber bump stop and gaiter.
Now undo the damper retaining collar, a professional citroen tool should be used for this but in the absence of this you can undo it using a pair of stilsons (an extension may be required to obtain the necessary force), make sure you do not damage the collar.
Once the collar is undone, remove it and pull the shock absorber out of the strut bottom. Slide the new shock absorber into the strut bottom with the aid of a small amount of grease then replace and tighten the collar.
Slide the rubber bump stop and gaiter back over the damper and replace the spring ensuring that it is positioned correctly on the spring cup.
Replace the upper spring cup, bearing and top mount, then replace the top nut (a new nut should always be used) and tighten by holding the damper with the T40 socket again
Release the spring compressors ensuring the spring remains seated correctly upon both upper and lower spring cups
Re-fit the strut to the vehicle.