It seems as though they are reviewing all new articles for the niche sites. So just publish high quality hubs and they should get recommended on their own to the site. Publish one at a time and wait for one to get reviewed before publishing the next one. That way you can use the feedback to make sure the following articles are up to par.
High quality images. Outgoing links must be very strong. Amazon products only if you have used it yourself and can provide a good reason for including it. Very high quality writing.