The first way is through the official forums, both via the individual realm forums and the multi-realm guild recruitment forum. Often times this means making a detailed and professional looking post and ensuring that it's bumped to first page of the forum several times a day.
The second way is through unofficial sites and forums. The process is similar to recruiting on the official forums, but recruitment generally is more directed towards the site's (and ideally the guild's) audience. For instance the guild recruitment forums on MMO-Champion are a great place to find hard core raiding guilds. Our Guildwatch column is another great tool.