It is a good idea to start with defining the constants in your life and to start to play with them. For example: eating, sleeping and showering are a constant in most people’s lives and it is a new experience for them when they are altered in the army.
Work to be able to work well and calm without one or two nights of sleep
Learn to manage without fully showering for at least three days and see how you can somehow clean the important parts
Learn to work without food or drink for 24 and than for 48 hours while you monitor your mood and performance. Look up how to notice if you are well in terms of water and energy and go from there. Taking care of yourself in any situation possible.