To perform this you will need the following tools:
4mm Allen Key
Small thin flathead screwdriver
Note: This process is easiest with the front fork completely removed from the bike or with the bike on a stand. If you do not have access to a stand or the fork is still attached to the bike this will be a difficult process.
Undo the bolt connecting the fender mount to the bottom of the fork.
Hold the bike fork vertically. Take your small screwdriver.
Tap the lowers of the fork with your hand. Then insert the small screwdriver to align the hole in the internal plate with the hole on the bottom of the fork lower. This may take some trial and error along with patience.
When you have the two holes aligned, insert the bolt and fender mount to the bottom of the fork. Make sure the fender mount is correctly orientated. Then fasten the bolt in by about half of it’s length - you will be able to tell that everything is aligned if the bolt is threading in and you don't feel anything moving.
With the bolt halfway inserted you can return the fork to a horizontal position and finish screwing the bolt in.