They can be mounted so that they wrap around the fork tubes below the triple clamp (the big clamp that the fork tubes come through) like in the picture below, from this month's Featured Monster
Sometimes, they are mounted so that they wrap around the part of the fork tube that sticks out above the triple clamp, like below. Note that the clip-ons below have a 'riser'. The riser actually clamps onto the fork tube (and raises up the bars), and the bars then go through the riser.
Here's one example of actual clip-ons, without a riser: