Using a 4mm Allen key, remove the old fender. Clean the area under the fender and between the front and rear triangles.
Fold the length of foam into a U-shape and slot it into the gap between the front and rear triangles.
Using the 4mm, install the new fender noting the slotted holes to allow the fender to be centered relative to the seat tube. Also note how the fender interacts with the derailleur cable housing.
Before fully clamping the fender down, it is key to ensure that the derailleur cable housing is pulled tight and follows a straight path from the top tube down to the driveside seat stay.