Here are the steps of how I mounted mine:
1. Wash both fender and extender with warm soapy water thoroughly. Then wipe the mating surfaces with rubbing alcohol. Mark a line on the fender of where the extender will go.
2. Roughen the mating surfaces with sandpaper. I used 220 grit this time, but anything between 80 and 400 would work. This step is crucial for giving the adhesive max surface area to grab onto.
3. Masking tape both sides to shield from glue overrun. Make sure you press down the edges tight, otherwise the glue might seep under the tape.
4. Apply adhesive of your choice. I prefer Polyurethane glue (Gorilla Glue) for several reasons:
a. Bonding strength is extremely strong.
b. Very lightweight.
c. It expands as it cures, filling gap and getting into all the nooks and crannies. This is especially important for this fender because there are good size gaps between the fender and the extender.
d. Once it is cured, it remains somewhat flexible, which provides a good bit of impact and vibration resistance.
5. Apply first application of glue sparingly. Then press the parts together to check where additional glue may be required. Apply second time to those areas. You want sufficient glue to coat all mating surfaces, but too much glue will squeeze out, overrun and make a big mess.
6. Press the 2 parts together. Once you are satisfied with the fit and alignment, tape the mating edges to "lock" in the fitment. Now clamp the two parts together. You want sufficient clamping to bring the mating surfaces as tightly together as possible, but not to much so as to warp and twist the parts out of shape.
7. Wait 6-12hrs before removing clamps. Curing time depends on ambient air temp and moisture content. When in doubt, give it full 24hrs.
8. Some glue expansion and overrun is a good sign that you have max glue coverage, right up to the mating line. Unfortunately, this also means taking off the masking tape can be rather tedious, as any glue overrun along the edges will cause the tape to be difficult to remove. Careful picking with a sharp hobby knife works well here. The tedium pays off with a clean, tightly bonded, lightweight, and rigid assembly.