It's real easy. Had to do the same when I added a passenger pad to my Street Bob... except it was the other way around... but it's still as simple as pulling out the plastic plug and puttin' the mounting hardware in the hole.
Quick tip: Get a long zip-tie (or something similar with a "stopper" on the end) and slide the mounting hardware on it. Make sure the top of the hardware is facing the skinny end. Feed that skinny end up under the fender and through the hole where you removed the cap. It can be a little tricky getting the pointy end of the zip-tie to find the whole when you've got your hand jammed under the fender, but have patience... you'll get it. Pull the zip-tie up through the hole until the top of the mounting hardware slips through the it. Pop on the plastic collar and Bob's your uncle.
Clear as mud?
Welcome to the forums and the family. You'll like it here... even if we are dysfunctional at times

Watch out for Texan. He's like the crazy uncle that always gets drunk and falls in the pool.