Fitting a mudflap to your front mudguard helps to keep your bike cleaner in the muddy conditions and it really helps to keep your radiator cleaner which helps prevent your bike from overheating.
Using a very sharp knife remove the lip on the underside of the bottom of the mudguard, cut straight down through the lip on both side like shown in the picture below, this gives something to cut up to and leaves a nice clean edge.
Keep the blade flat while removing the lip to help stop you digging into the rest of the mudguard, also it can make it easier to cut by taking lots of thinner slices rathe than trying to cut it all off in one go.
This is what you want the mudguard to look like before sticking the flap on. Note the lip is totally gone on the bottom edge only, ideally the cut surface wants to be flat and in line with the rest of mudguard area where you will be sticking the flap.
If mudguard has got mucky while cut off the lip give it another wash or wipe it down with brake cleaner or isopropyl alcohol, let it dry and the remove the backing paper off the adhesive on the flap and stick it in place. The flap should line up with the V shape lip on the underside of the mudguard as shown below.