Soldiers were encouraged to make a will once they joined an operational unit, usually straight after completing their initial training. There was no compulsion. The Army provided simple forms* for the purpose and the rules for Army wills are much more flexible than ordinary wills. For example a soldier under 18 years of age can make a serviceman's will, but a civilian of the same age couldn't.
*Today the equivalent is called the MOD Form 106 and is used by all three services.