It depends on many things: what extender is made of, what fender is made of, what the length of the extender is. I was lucky in my case i just used the same bolt that holds fender. At my another bike, i had to drill two holes to attach it by two bolts. You should think of something that fixes it well and is reliable. For rubbery extenders i would recommend big washers.

And yes, i saw attachment by adhesive tape. Can work, but you should understand how long and how well.