The Freeform training is unlocked after you complete the Guided training. It can be found by navigating to the campaign section and scrolling right. There will be two kinds of training, one labeled Guided and one labeled Freeform. The Freeform training allows you to use any method of execution on the target. You can use a garrote wire, explosives, knives, the Guided training method or guns. Getting detected is fine as long as you escape and it doesn't interfere with your preferred objectives. You can also disguise yourself as Mr. Norfolk for the execution by luring him into a back alley with coins and knocking him out, which works really well and gets you the following achievement: