Learn Spanish (Spain): Ya está todo; falta que firmes aquí. - Everything is ready; all that's left is for you to sign here.

While Todo está listo is grammatically correct and means "Everything is ready", native speakers frequently use Ya está idiomatically to indicate that a task or process is complete. Adding todo emphasizes that absolutely everything is finalized.

Why is firmes in the subjunctive rather than the indicative?

The phrase falta que acts as an impersonal expression showing that an action is pending or required. Like other expressions of necessity (such as es necesario que or hace falta que), it triggers the subjunctive for the following verb.

How exactly does falta translate to "all that's left is"?

The verb faltar literally means "to lack" or "to be missing". Therefore, falta que firmes translates literally to "it is missing that you sign." In English, we express this same idea naturally by saying "the only thing left is for you to sign."

Could I just use the infinitive here and say falta firmar?

Yes, you can use the infinitive, but it changes the meaning slightly. Falta firmar is general and means "it needs to be signed" without specifying who should do it. By using que plus the subjunctive firmes, you are specifically telling the listener that they are the one who needs to sign.