If you intend to mount a road sign to a new or existing post, you'll need road sign clips.

Road signs for post mounting come with channel section on the reverse.

Clips come in pre-determined diameters for circular posts. One end of the clip slides into the channel, the other is then bolted in place around the post. 

Mounting road signs to posts with channel section clips is very easy and only requires a spanner for an M6 bolt.

Road sign clips priced per single clip.