Typically you would reach out to bloggers asking if they are interested. Find enough interested parties, and then have a default post including images and all information that you would like for your giveaway. Then coordinate the date that you would like them to post it. If you have a strong network of friends blogging, that is a good way to start. Just make sure to understand that them hosting it for you is taking up space on their blog too, so you will want to make sure the post is something they are going to want to promote, and also maybe provide some fun incentive for blogs that participate.