You need to hit level first, which will unlock the Rats mission available from the Crime.net map.
Go through the mission until you reach a side mission called Bus Stop. What you will need to do is kill all of Mendoza's men on the bus.
Do it (should take you less than 40 seconds) and then run back to the heli.
This will trigger the end of the mission and you'll get your rewards (XP and money).
Now, when it says "Payday" press A to display the cards and choose one to get a random item. Then, when the item is shown, press Start and select Restart Game (this is the cheat!).
You will have the Bus Stop mission reloaded then and can do it all again (super quick) and get the rewards all over again! You even get to keep the Payday item.
Once you've gone from level 13 to level 50, go back to the Crime.net map and look for the Rats mission with three yellow stars. Select this version, then do the above trick all over again through to level 100.
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