You would need to fill out an Application for Pre-need Determination of Eligibility for Burial in a VA National Cemetery VA Form 40-10007 and sign it.  You would sent that completed application with copies of your supporting documents to the National Cemetery Scheduling Office in St. Louis.
Can my sister who is 17 and currently serving now apply for burial using the same method you are telling me?
If a person is under 18, mentally unable to make decisions for themselves, or physically unable to sign the form they can have a court-appointed representative to sign the form for them.  However, if your sister is active, she is not a veteran. She does not yet know the terms of her discharge.
Does this also pertain to my own adult children?