Embassy legalisation for use in the United States
The United States accepts a single-step certification, so the consular stage does not run and no American mission needs to see your paper. One certificate from the FCDO Legalisation Office completes it. Saying that at the top matters, because a request letter often reads as though a chain of offices is waiting, and readers act on the letter rather than the reality. There is one office in this job, and it works by post.
Reading the request you were sent
Start by underlining three things in the letter: which record they want, whose name must be on it, and where it has to arrive. Everything else in the wording is usually inherited text. A demand for document legalization for United States purposes describes an outcome, not a route, and the outcome is a record whose signature has been officially confirmed. Get those three details right and the rest of the job is mechanical.
The Belfast to United States route in practice
You order a certified copy of the entry, post it to the Legalisation Office, and receive it back with an official certificate attached. That is the point at which the paper is finished and ready to cross the Atlantic. Since embassy legalization for United States from Belfast is not a stage that exists, nothing waits on a diplomatic appointment and no file sits in a consular tray. The only variable left is the post.
What it costs and where the paper ends up
An official fee applies at the Legalisation Office, plus whatever the register charges for the certified copy and the cost of postage each way. No consular fee arises, since no consulate handles the file. Keep the entry you already own at home and let the fresh copy travel, because a copy can be reordered and a lost original starts the whole request again. The paper that crosses the Atlantic should always be the replaceable one.