Well, it turns out that they didn't. The plan includes a couple hundred billion in specific cuts (more on that later), but where does the rest come from? It comes from hand-waving, of course. Over $2tn of the spending cuts are merely a promise to keep funding levels for discretionary spending at 2006 levels:
Like most major spending cut proposals, this one's not entirely rigorous. It relies principally on an aspirational spending cap -- specifically, limiting non-defense appropriations totals to their 2006 levels without adjusting for inflation. In other words, it punts the question of what to cut to future Congresses, which could just as easily bust the cap.
In other words, only a couple hundred billion is actually specified and could conceivably take effect. There's no way that future congresses will stick to this cap. If nothing else, the United States of America is GROWING, so without increasing funding, these programs will have to do less for more people.
The actual cuts seem to be a list of conservative bogeymen, including, apparently, getting rid of USAID entirely. Apparently, the military is immune from cuts, but USAID should go away. Andrew Exum has an excellent takedown of the whole idea:
Sec. Gates will argue, supported by veterans from Iraq and Afghanistan, that while USAID has problems, the money we spend through it is just as related to U.S. national security interests as the money we wasted on the Crusader or the money we spend to put an 18-year old through basic training. To not understand that is embarrassing because it means you're an elected policy-maker and still uneducated about the wars we've been fighting for almost 10 years now.
Also included is elimination of a program that provides funding for family planning services for the poor. Hey, that one hits two GOP priorities: screwing the poor and continuing to have an archaic and counterproductive attitude toward sex and reproduction.
In short, yay, there are actual cuts to criticize now. They amount to about $200bn over 10 years, which is approximately how much the Pentagon spends on paperclips. Because, of course, they only hit non-security discretionary spending and that is just not where the money is. The other $2.3tn? Well....