Not sure about what has changed on the exterior since 1999 but here goes:
By the way, if you have the sport model, I don't think the flared fenders will accommodate the mud flaps.
I'm assuming the double-stick adhesive tape is provided...that is used to attach the front mud flap to the outside lip of the fender. There should be factory screw holes on the inside of the fender lip. Line those up with the mud flap before attaching the taped side to the fender. I believe there are at least two sheet metal screws to attach near the lower part of the fender lip and one (or two) clips near the top.
The rear flaps go on pretty much the same way, but the curvature and mounting location precludes the use of the sticky tape and clips for the rears. I hope they've improved the tape, since mine have let go (off to Home Depot for some 3M sticky tape).
Hope my instructions are accurate enough to help out...good luck!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car