The answer depends on the nature of the content and the goals of the person who donated the 300 SP. The structure would be different for a sponsor seeking to build a community, a sponsor with a product to sell, a group promoting an event or cause.
There are many other questions, like is the content to be published somewhere?
SteemIt makes things more interesting because people often upvote each other's posts. It is possible to kill a contest by judging posts by upvote count as people will hire bots and won't vote for other contestants.
The contest really needs to be designed around the desires of the sponsor and the nature of the content.
Thank you @yintercept for your mature comment
This is such a good point.
Thaks for sharing. Yours, Piotr