I get asked that a lot. CNN has provided a very nice interactive detailing how the FSG's work.
I add that judges find sentencing factors by only a preponderance of the evidence; hearsay is allowed at sentencing; and the defendant has very limited cross-examination "rights."
Generally, a probabation officer files a pre-sentencing report that serves as the basis for the sentence. PO will come to defense lawyer's office to interview client. PO will then state what Guidelines she thinks applies. USA's and defense lawyers will fight with each other over what section of the USSG applies - faxing obnoxious memos back and forth to each other. Then judge usually accepts the PO's factual findings (which are not subjected to X-E), and listens to lawyers give reasons why some section does or does not apply.
Sentencing arguments must more closely resemble civil law countries' system of law. You do not (generally) have an argument, unless you can cite it in the code.