Start by buying as many lockpicks as you can (the more, the better.) Once you have about 500-600 lockpicks, claim a survival package that is dropped off by plane. You only need one package to be effective. Once you have the package in your inventory, visit a safe house.
At the safe house, access your inventory bag. Select all the lockpicks and deposit them into the bag. After the lockpicks left your personal inventory, highlight the lockpicks stored in the storage bag. Next press
&
simultaneously. At this point it should trigger you outside of the inventory bag and have the selection menu on screen. At this point, dont press any face buttons and just stand there until the menu fades. Once it does, enter the inventory bag menu and, only using the directional buttons, scroll down to the package you retrieved from the plane and hit
(all). It should duplicate the package to match the amount of lockpicks you had.
After you collect all of the packages into your inventory, deliver them to the quartermaster and you should go from whatever level you were to level 25 depending on how many lockpicks you had.
S/N you can use this method on just about everything minus weapons and guns.