Since the expected veto of the Iraq spending bill is so high profile, how does a president actually do it?
The answer comes from the U.S. Constitution, Article I, Section 7. It says if the president wants to veto a bill, "he shall return it, together with his Objections, to the House in which it shall have originated, who shall enter the Objections at large on their Journal, and proceed to reconsider it."
As a practical matter, say administration officials, it works this way. The president doesn't actually do anything to the bill. No rubber stamps that say "VETOED" across the face of it, as visually appealing as that would be. Instead, he writes a message to, in this case, the House, which states his objections. This is the veto message. That, along with the bill, will be returned to the House by the executive clerk.
Because of the Constitution's requirement that the bill be returned to Congress, it's not truly vetoed, in a legal sense, until it's delivered to the House -- even though it's commonly considered vetoed as soon as the president signs the veto message.
Oddly, the Constitution does not use the word "veto" to describe this process, and the word never appears in the text.