I recently purchased the 312mm fronts and 256mm rear brakes from a Golf V6 4motion to fit on my VRS.

From reading the other posts I see there is a problem with the rear carries not fitting properly due to the 4wd system.

Can anyone tell me what I need to do to fit them properly? I'm reluctant to get another set of rear brakes now that I have these ones.

Also any advice on what to do about the rear splash guards?

Not sure on splash guards (thought they were stone guards) but on the UK MkIVs forum I am sure someone was making the adaptor bracket to fit the 4mo rears to a 2wd car.  I bought them to use with an S3 set of rear brakes and they still didn't fit so I took easier, safer option of buying octy vRS rears.

Sounds like you just want a set of carries to suit the rear 256mm setup, obviously nothing 4wd haha. have a flick around ebay. 

I just had a look on that UK MkIVs site and it says the following:

 

• The standard 232mm Solids can be upgraded for the 256mm Vented setup, the bigger carriers & calipers just bolt straight on if they come from a 2WD car. If the calipers & carriers have come off a Quattro/4 Motion then you will need some 6mm spacers.
• 
  
• You will need to modify your standard splash guards as they curl in over the top of the std disc, and would catch on the new, larger disc
  
  
  I might try that and see what happens