Series is a group of 6 or more cards that share a function or a theme. Best way to make a series is to choose a specific game mechanic or a theme, and then design 12 cards (one card from each element) using that mechanic or theme. You can also design more than 12 if you so choose.
Once you finish at least 6 cards belonging to a series, you can start a series thread which explains the idea behind that series and also introduces all the cards belonging to that series. Cards that belong to a series, will still have their own individual threads just like all the other cards. Series thread is just a "bonus" that helps community members and developers find other cards in your series more easily.
If your series is one mechanic identical across all cards then put them all in the one idea submission. A good example of this is Pillars/Towers which all belong to different elements but are mechanically identical.
Currently there is no template for series threads. You can pick the look and layout yourself. Once you've made a series thread, you can post here or PM a Card Curator to move it here.
Please note that Series topics are different than Portfolios. Please read this topic if your portfolio has been placed in the Series topic temporarily:
http://elementscommunity.org/forum/index.php/topic,24750.0.html