Finding a Borgia Courrier is half the battle as you also need to catch them
Firstly do not get these confused with the men running around on the map with the red purse icons, these are similar but not the men you are after.
The Borgia Courriers do not
appear on your map and appear randomly, ive had them mostly show up in Florence
and will be denoted by the white arrow on their heads and will usually scream 'Murderer! stay back' once they see you.
Once you have found a Courrier you need to target them with
and chase them (
) though they are hard to catch as they love roof tops.
Once you are close enough you can either tackle them (
) or assassinate them (
Here's a video made by RavenXGamging, skip to 1:54