you just put the link they give you inside you sig. area and do your usual postings?
yes, but some campaigns ask you to make at least 20 (or 15, or 30) posts weekly
sometimes you have to put theirs ad text not only in signature but also in your personal message (see mine for example)
I will receive 5000 satoshi for this comment:)
join the campaign if you want
https://bitcointalk.org/index.php?topic=751056.0