And thats if those posts are approved, which most dont go through since its counted as spam.
Just make sure that you aren't writing one-liners, or anything else listed in the campaign, and you're pretty much set.
Now sometimes it will be counted as spam even if it's good, and you just have to grin and bear it. Don't complain, as most times that will get you a bad rep with the owner of the campaign. It's better to be on good terms.
Now that being said, it's rather challenging to actually get into a campaign, and I wouldn't exactly say it's challenging either. Just look around, chances are there is at least one place accepting.