If you think this is a bad guide and vote negatively, please post a comment or sent me a message why. I edit this a lot. I just want to make sure that this guide covers every detail so even the most noobiest player can get the achievement.
This can be done on ANY online mode (So preferbly pick the solo or the invite only online mode)
Then you are able to rob Stores. They show up on the map after / during the turtorial as a shopping tray icon.
Once you've started robbing a store by pointing a gun on the storeclerk you'll get a wanted level. Make sure to fill the intimidation bar and then pick up the bag filled with money they drop. Then either keep going robbing more stores, evade or die it's all irrelevant towards progression of the achievement.
But what is relevant is that the store icon will disappear once you either:
- Robbed it succesfully
- Failed it (by dying in the store before you get some money)
- If someone beat you to it making it unavailable for you to rob again the same in-game day.
If succesful the icon will later reappear with a check, indicating if you've robbed it. (or not, if unsuccesful)
So just look on your map for their locations and go rob all 20 to unlock your achievement. For some however the achievement seemed to pop after robbing only 18 or 19 stores. As shown in the comments below
- If you assault the storeclerk you get the option of taking out the money (with right on the d-pad) which to my opinion is faster, not sure if this is more profitable
- If you get away from the cops instead of dying you will get some xp
- Dying will cost you money and sometimes a bit of ammo so it's up to you how you want to get rid of wanted level. However you can put money you earned it the bank through an ATM or your phone to store it until you use it.
- The more stores you rob consectively, the higher your wanted level will jump to.
- You can check how many stores you've robbed (I do not know if this counts for unique stores) in the stats menu when you are online under the crimes tab
-Sometimes an NPC thug will start shooting at you, make sure to kill both the clerk FIRST and then thug or you will die and fail robbing the store
-Sometimes the clerk will pull a gun on you, then just go with the assault method described above
Credit to several people for several things, posted in the comments below!