Sprint.ly doesn't formally have the concept of "sprints." We've instead found a flexible planning workflow with the use of tags, filters, and the Timelines view. You may find info on how to create and track sprints/iterations in this related knowledge base article.
KB Article: Creating and tracking sprints/iterations
KB Artcicle: Estimating items & measuring velocity
KB Article: Philosophies used to build Sprint.ly
Video: Basics of Timeline View