Size is very important, from there, just do your best. If you are proud of the work you did, the other swappers will be as well. It's about being fun and sharing, not being perfect.
I would go easy on the starch because many people really don't like working with starched blocks. Also it's best not to send your block wrapped up with a scented dryer sheet, covered with cat hair, or smelling like stale cigarettes.