I just completed a very long but successful process of fitting the top and middle (or lower) grill parts to my 2005 Mini One.
The 1st part of attaching the top grill is a straight swap as suggested by many other posts. The use of the correct spanner and a flathead screwdriver easily remove the 12 screws that hold the grill in place. Once the screws are removed the grill is easily deattachable. The new black
JCW grill then simply slots into the original holes and secures its-self. This took about 20-30 mins as some of the screws holding the original grill are annoying to get a hold of.
The second stage was to attach the middle or lower part of the grill set. I joined Mini2.com to try and find some answers as to how to remove the chrome lower grill; i found that nearly all suggested to take the bumber cover off to achieve this. at this point i was like what?! Im not an expert put it that way. Anyway i found that the lower grill came in two parts - firstly the bottom part removes quite easily as no screws or adhesive is used again i used my trusty flat head screwdriver to carefully remove it. The top part had adhesive tape from one side of the grill to the other. Some force is needed; tbh i was a tad nervous when prying it off but once inital contact had been broken you could easily slide the lower grill part 2 by sliding the screwdriver underneath and at the same time pulling the grill off. This took about 45 mins as you want to be carefull not to damage the bumper.
To fit the
JCW on to the Mini i needed to create new slots as the
JCW was not designed to go onto a regular bumper. First taking time to meaure where the new slots need to be is crucial. Next by using a rubber headded hammer and again the mighty flathead screwdriver make rectangular slots about an inch across and roughly 1.5cm high. It took a bit of adjusting to get the slots the right size so the grill would fit in snug and flush to the car. After about another hour i put the lower grill on completely secure and flush and it looks sweet and pretty professional if i do say so myself

with almost no damage to the bumper or the grill.
I will get some pics up soon of the finished work...