1) BUMPERS are a straight fit, refer to Haynes manual to see where all the bolts are for fitting and removal.
2) ARCHES, put a dot of paint on the end of the clips so you'll see exactly where to drill into the front and rear panels (you can just bond the VT arches on if you want to) Some of the clips will fit into the original holes and will be apparent when you line up the arches.
Bare in mind that drilled holes may still not be 100% perfect and arches may not fit flush on their own (Also dont drill holes too big- do them slightly smaller than the clips else they wont stick into the bodywork) A better way is to use BOLTS instead of clips on the top part of the arches that go over the wheels, the head of the bolt slips in where the clip would, then put the thread through the hole and tighten up the nut on the inside of the arch until it sits fully flush against the bodywork.
You could/and will need to also apply some tiger seal (best around bottom edges of rear arch and where it reaches the door frame, be careful though as it can be messy) Use clamps to secure the sealed bits tightly against the bodywork (Remember to put something between the clamp and bodywork so that your paint doesnt get damaged)
As yours is flat arch aswel you'll need to cut away some of the front and rear quarter panels as it pertrudes below the VT arch. trial fit the arch first and mark with pen or paint where you need to cut up to.
Once cut make sure you treat exposed metal with rust protection and a bit of underseal after. This will ensure that you encounter no corrosion problems.
3) SIDESKIRTS you'll need to drill holes again on the top part which should be lined up again using the paint technique, id strongly advise sealing these aswel and clamping them where possible. Then underneath you can buy clips from citroen to attach, Or if you'd prefer it is possible to screw the bottom part onto the car, remember to treat the area for corrosion aswel.
4) DOOR MOULDINGS, the top half of these you'll find that the clips fit directly into the original bump strip holes. Around the bottom and sides of the moulding, again use tiger seal and clamp it to the door for 24 hours (obviously ull need to put the car in a garage overnight if necessary)
This should complete the fitting of your VT kit, get a friend to help you if possible to double check any areas that you may have missed and to help secure bits in place-especially when trying to do the sideskirts.
Remember to treat any cut or drilled areas for rust!
Please note that you could also just tigerseal the whole kit, this is the less time consuming way and its upto your own discretion on whether you think its safe to do it that way, personally id rather do a job properly.