The illustration inside of the hair dress should be manifold and not have any stray pieces. If your artwork is complex with multiple objects, you should merge them together to create one unified vector path. Your artwork will need to be exported in the SVG file format.
Stuck on the creative stuff? You can get some great inspiration and actual vector files on sites like http://vecteezy.com and http://all-free-download.com. The design should complement your event or character for a great cosplay!
When your design is finalized, you will want to flip the design up-side-down so that the artwork is printed facing the heated build plate. The side that gets printed on the heated surface creates an almost seamless texture that makes for a great quality print.
For extra awesome, import separate SVG artwork to create depth and accessories to the hair dress for an epic design! To get familiar with tinkercad's UI, check out their tutorial videos on their youtube channel. It's an easy and powerful CAD web app!