Each of the magazines has it's own set of guidelines for submissions. You can generally find this information on the magazines website, or amongst the pages of a physical magazine. Read through the instructions carefully. Some publishers require you to send the jewelry in to them before they decide to use your pieces. Others only require you to send good quality digital images via email to make a preliminary decision.
One thing that is important, is practicing writing instructions. Depending on the publication, you will also have to study how the instructions are written for each. Some require you to write in more details, and some require certain wording when describing techniques.
Create designs that will stand out from the crowd, and definitely try to submit more than one design. Although some designs might be rejected, the more you submit, the more of a chance you have at getting some accepted. It's also smart to submit earrings or bracelets that match your submitted necklace designs. Sometimes publications like to showcase these together, and sometimes separately but within the same issue.
Here are some links to some of the popular beading magazines and their submission guidelines: