So how do you join in? First, you must have a completed and polished (this means no first drafts!) manuscript that you feel is ready for submission. Next, come up with several great 140 character pitches to post on Twitter throughout the day. Then, watch for agents and editors to favorite your pitch. If they do, check their profile for a way to submit to them (they will likely have tweeted details on how to that day, if not, click on their website link). A favorite means you are invited to submit. That means, if you aren't an agent or a publishing house editor, do NOT favorite writer's tweets. You can retweet them, but don't favorite them as that is only for requests. There are other hashtags besides #PitMad that you will need to use. More details here.
Since pitches are so short, it doesn't hurt to have a link on your Twitter profile to where agents and editors can read more about you and/or your manuscript. I love to see samples of an author's writing somewhere on their website or blog, even if it isn't samples of what they're pitching.