Fitting these is fairly easy, if potentially a bit mucky. It's much easier if each wheel is removed to provide easy access, though it's only really essential for the rear wheels.
If buying from a dealer, ask them to print out the fitting instructions - that's part of the service you are paying for, even if just buying them.
At the rear particularly, you must scrupulously clean the back plastic where you will be sticking on the edge of the mud flaps. I use meths to get them spotlessly clean - plus you can take a swig or two if it all gets a bit too much.... I didn't clean my previous Mini's trim quite well enough and the rear flaps began to lift a bit. It should now be warm enough for the sticky tape to stick, but if the cold returns, use a hot air gun (or even a hair-dryer) to get the black trim and mud flap toasty warm before sticking them together.
There is a trick to fitting the inner fastener on the rear mud flaps. Fit this as the first fastener, so that you can peel back the wheel arch liner and hold the backing plate in place - once the mud flap is in place, this isn't accessible unless you have three wrist joints on one arm (and eyes on your knuckles to see where it goes)....