The video above is great for showing you what to do when you get mugged in order to complete the achievement requirements, my problem however was getting mugged in the first place (I hadn't ever been in 5 days of total play time.)
I used the in game phone to message a random player in the same session as me (who is over level 50) and asked them to mug me (make sure you have cash on you, not just bank). I got a response on my second try, waited around with a gun drawn and had the trophy in about 10 minutes.
Hope this helps.