Here is the spread webinar section from the forums. All of the webinars regarding spreads are in there. A lot of them will be application type where they match the spread to a strategy. For the basics, these are what I would recommend:
5 Steps To Picking A Nadex Spread
How To Pick The Best Nadex Spread
How To Trade The Right Nadex Spread
Matching The Right Spread To The Right Trading Strategy
Hope this helps.
Happy trading!!! Brad