There is no way that Trump can average $1 trillion per year in spending reductions without deep cuts to Medicare, Medicaid, Social Security, and other mandatory spending from the very start.
"The federal government devotes roughly one-sixth of its spending to 10 major means-tested programs and tax credits, which provide cash payments or assistance in obtaining health care, food, housing, or education to people with relatively low income or few assets."
As of Fiscal Year 2011 total spending on those programs reached close to $1 trillion
One thing Trump could do immediately, is take all those programs, currently scattered among various agencies, and bring them all under one roof. Then fire the federal employees thus shown to be redundant.