Do the first mission.

Next, hire your created soldiers. Save the game.

Go to the game install directory: Steam\steamapps\common\XCOM 2\XComGame\Config.

Open the DefaultGameData.ini

Look for:
XComHeadquarters_DefaultTrainRookieDays[0]=????? ;Easy
XComHeadquarters_DefaultTrainRookieDays[1]=????? ;Normal
XComHeadquarters_DefaultTrainRookieDays[2]=????? ;Classic
XComHeadquarters_DefaultTrainRookieDays[3]=????? ;Impossible

Replace the "?????" with whatever number of days you want.

Go to the guerilla training school and train your customized characters in the class of your liking. This will take however many days you put into the "?????" section. The lower, the faster.

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