A. Every agent is different in terms of what they want, so there is no preferred method, so to speak. Most will probably tell you to send the proposal right away. Some will want to see a strong query letter, and then ask for the proposal if they’re intrigued by the query. Much more often than not, they will say exactly what they want on their website. If they do NOT (and you’ve really looked everywhere), then I advise just sending the proposal. Most agents can size up a proposal in about two minutes. After that, they will either be interested and read the whole thing, or they will send a form rejection your way.
And to address another point here, the proposal is indeed sent to publishers by the agent, but an agent will go over it with a fine-tooth comb for a while to make sure it’s perfect.