Installation isn't difficult, just plop the text file somewhere nice and follow the instructions for the tool.
EW only.
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In normal play, when a Rookie is promoted, the game gives them their base perk automatically. This mod prevents that; instead, while waiting to assign the perk, you can use the soldier-scrolling buttons on the bottom of the Squaddie's "Abilities" page to change their class.
Does not work on MECs, or once the first perk is locked in, but should work with reward soldiers (unique or otherwise). When changing class, the unit being displayed will not update immediately - so it might look like your new Assault is still holding an LMG, but they aren't. Changing any equipment will sync it up. Also note that a soldier just switched off of Heavy won't have a pistol equipped automatically, but be left with a placeholder empty slot. I'd recommend giving them a proper sidearm pronto.