The first step in how to translate documents correctly is deciding what actually belongs in the translation set. That sounds obvious, but it is where many preventable mistakes begin. For USCIS, the official rule in 8 CFR 103.2(b)(3) applies when you submit foreign-language documents. That means the trigger is not whether the document looks important to you. The trigger is whether the page contains foreign language and whether you are submitting it as evidence. If the answer is yes, plan for a full English translation.
This is especially important with common packet documents such as /documents/birth-certificate, /documents/passport, and /documents/transcripts. People often think only the front page matters, then discover later that a back-side legend, a seal, a registry note, or a handwritten annotation should have been translated too. A complete document translation process starts by collecting the full set first: front, back, attachments, annotations, and any pages you might submit if the receiving authority asks for them.
The practical question to ask is simple: "Will the reviewer need this page, note, or stamp to understand identity, status, relationship, education, or history?" If yes, keep it in scope. If you are working on a family-based filing or adjustment packet, the use-case pages at /immigration/uscis and /immigration/green-card can help you identify the most common supporting records before you order.